Means and method for the detection of cardiac events

ABSTRACT

Disclosed is a system for the detection of cardiac events that includes an implanted device called a cardiosaver, a physician&#39;s programmer and an external alarm system. The system is designed to provide early detection of cardiac events such as acute myocardial infarction or exercise induced myocardial ischemia caused by an increased heart rate or exertion. The system can also alert the patient with a less urgent alarm if a heart arrhythmia is detected. Using different algorithms, the cardiosaver can detect a change in the patient&#39;s electrogram that is indicative of a cardiac event within five minutes after it occurs and then automatically warn the patient that the event is occurring. To provide this warning, the system includes an internal alarm sub-system (internal alarm means) within the cardiosaver and/or an external alarm system (external alarm means) which are activated after the ST segment of the electrogram exceeds a preset threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.10/853,058, filed on 26 May 2004 now U.S. Pat. No. 7,558,623.

FIELD OF USE

This invention is in the field of systems, including devices implantedwithin a human patient, for the purpose of automatically detecting theonset of a cardiac event.

BACKGROUND OF THE INVENTION

Heart disease is the leading cause of death in the United States. Aheart attack (also known as an Acute Myocardial Infarction (AMI))typically results from a thrombus that obstructs blood flow in one ormore coronary arteries. AMI is a common and life-threateningcomplication of coronary heart disease. The sooner that perfusion of themyocardium is restored (e.g., with injection of a thrombolyticmedication such as tissue plasminogen activator (tPA)), the better theprognosis and survival of the patient from the heart attack. The extentof damage to the myocardium is strongly dependent upon the length oftime prior to restoration of blood flow to the heart muscle.

Myocardial ischemia is caused by a temporary imbalance of blood (oxygen)supply and demand in the heart muscle. It is typically provoked byphysical activity or other causes of increased heart rate when one ormore of the coronary arteries are obstructed by atherosclerosis.Patients will often (but not always) experience chest discomfort(angina) when the heart muscle is experiencing ischemia.

Acute myocardial infarction and ischemia may be detected from apatient's electrocardiogram (ECG) by noting an ST segment shift (i.e.,voltage change) over a relatively short (less than 5 minutes) period oftime. However, without knowing the patient's normal ECG patterndetection from standard 12 lead ECG can be unreliable. In addition,ideal placement of subcutaneous electrodes for detection of ST segmentshifts as they would relate to a subcutaneously implanted device has notbeen explored in the prior art.

Fischell et al in U.S. Pat. Nos. 6,112,116 and 6,272,379 describeimplantable systems for detecting the onset of acute myocardialinfarction and providing both treatment and alarming to the patient.While Fischell et al discuss the detection of a shift in the S-T segmentof the patient's electrogram from an electrode within the heart as thetrigger for alarms; it may be desirable to provide more sophisticateddetection algorithms to reduce the probability of false positive andfalse negative detection. In addition while these patents describe somedesirable aspects of programming such systems, it may be desirable toprovide additional programmability and alarm control features.

Although anti-tachycardia pacemakers and Implantable CardiacDefibrillators (ICDs) can detect heart arrhythmias, none are currentlydesigned to detect ischemia and acute myocardial infarction eventsindependently or in conjunction with arrhythmias.

In U.S. Pat. Nos. 6,112,116 and 6,272,379 Fischell et al, discuss thestorage of recorded electrogram and/or electrocardiogram data; howevertechniques to optimally store the appropriate electrogram and/orelectrocardiogram data and other appropriate data in a limited amount ofsystem memory are not detailed.

In U.S. Pat. No. 5,497,780 by M. Zehender, a device is described thathas a “goal of eliminating . . . cardiac rhythm abnormality.” To dothis, Zehender requires exactly two electrodes placed within the heartand exactly one electrode placed outside the heart. Although multipleelectrodes could be used, the most practical sensor for providing anelectrogram to detect a heart attack would use a single electrode placedwithin or near to the heart.

Zehender's drawing of the algorithm consists of a single box labeled STSIGNAL ANALYSIS with no details of what the analysis comprises. His onlydescription of his detection algorithm is to use a comparison of the ECGto a reference signal of a normal ECG curve. Zehender does not discussany details to teach an algorithm by which such a comparison can bemade, nor does Zehender explain how one identifies the “normal ECGcurve”. Each patient will likely have a different “normal” baseline ECGthat will be an essential part of any system or algorithm for detectionof a heart attack or ischemia.

In addition, Zehender suggests that an ST signal analysis should becarried out every three minutes. It may be desirable to use both longerand shorter time intervals than 3 minutes so as to capture certainchanges in ECG that are seen early on or later on in the evolution of anacute myocardial infarction. Longer observation periods will also beimportant to account for minor slowly evolving changes in the “baseline”ECG. Zehender has no mention of detection of ischemia having differentnormal curves based on heart rate. To differentiate from exerciseinduced ischemia and acute myocardial infarction, it may be important tocorrelate ST segment shifts with heart rate or R-R interval.

Finally, Zehender teaches that “if an insufficient blood supply incomparison to the reference signal occurs, the corresponding abnormal STsegments can be stored in the memory in digital form or as a numericalevent in order to be available for associated telemetry at any time.”Storing only abnormal ECG segments may miss important changes inbaseline ECG. Thus it is desirable to store some historical ECG segmentsin memory even if they are not “abnormal”.

The Reveal™ subcutaneous loop Holter monitor sold by Medtronic uses twocase electrodes spaced by about 3 inches to record electrocardiograminformation looking for arrhythmias. It has no real capability to detectST segment shift and its high pass filtering would in fact precludeaccurate detection of changes in the low frequency aspects of theheart's electrical signal. Also the spacing of the electrodes it tooclose together to be able to effectively detect and record ST segmentshifts. Similarly, current external Holter monitors are primarilydesigned for capturing arrhythmia related signals from the heart.

Although often described as an electrocardiogram (ECG), the storedelectrical signal from the heart as measured from electrodes within thebody should be termed an “electrogram”. The early detection of an acutemyocardial infarction or exercise induced myocardial ischemia caused byan increased heart rate or exertion is feasible using a system thatnotes a change in a patient's electrogram. The portion of such a systemthat includes the means to detect a cardiac event is defined herein as a“cardiosaver” and the entire system including the cardiosaver and theexternal portions of the system is defined herein as a “guardiansystem.”

Furthermore, although the masculine pronouns “he” and “his” are usedherein, it should be understood that the patient or the medicalpractitioner who treats the patient could be a man or a woman. Stillfurther the term; “medical practitioner” shall be used herein to meanany person who might be involved in the medical treatment of a patient.Such a medical practitioner would include, but is not limited to, amedical doctor (e.g., a general practice physician, an internist or acardiologist), a medical technician, a paramedic, a nurse or anelectrogram analyst. A “cardiac event” includes an acute myocardialinfarction, ischemia caused by effort (such as exercise) and/or anelevated heart rate, bradycardia, tachycardia or an arrhythmia such asatrial fibrillation, atrial flutter, ventricular fibrillation, andpremature ventricular or atrial contractions (PVCs or PACs).

For the purpose of this invention, the term “electrocardiogram” isdefined to be the heart electrical signals from one or more skin surfaceelectrode(s) that are placed in a position to indicate the heart'selectrical activity (depolarization and repolarization). Anelectrocardiogram segment refers to the recording of electrocardiogramdata for either a specific length of time, such as 10 seconds, or aspecific number of heart beats, such as 10 beats. For the purposes ofthis specification the PQ segment of a patient's electrocardiogram isthe typically flat segment of a beat of an electrocardiogram that occursjust before the R wave.

For the purpose of this invention, the term “electrogram” is defined tobe the heart electrical signals from one or more implanted electrode(s)that are placed in a position to indicate the heart's electricalactivity (depolarization and repolarization). An electrogram segmentrefers to the recording of electrogram data for either a specific lengthof time, such as 10 seconds, or a specific number of heart beats, suchas 10 beats. For the purposes of this specification the PQ segment of apatient's electrogram is the typically flat segment of an electrogramthat occurs just before the R wave. For the purposes of thisspecification, the terms “detection” and “identification” of a cardiacevent have the same meaning. A beat is defined as a sub -segment of anelectrogram or electrocardiogram segment containing exactly one R wave.

Heart signal parameters are defined to be any measured or calculatedvalue created during the processing of one or more beats of electrogramdata. Heart signal parameters include PQ segment average value, STsegment average voltage value, R wave peak value, ST deviation, STshift, average signal strength, T wave peak height, T wave averagevalue, T wave deviation, heart rate, R-R interval and peak-to-peakvoltage amplitude.

SUMMARY OF THE INVENTION

The present invention is a system for the detection of cardiac events (aguardian system) that includes a device called a cardiosaver, andexternal equipment including a physician's programmer and an externalalarm system. The present invention envisions a system for earlydetection of an acute myocardial infarction or exercise inducedmyocardial ischemia caused by an increased heart rate or exertion.

In the preferred embodiment of the present invention, the cardiosaver isimplanted along with the electrodes. In an alternate embodiment, thecardiosaver and the electrodes could be external but attached to thepatient's body. Although the following descriptions of the presentinvention in most cases refer to the preferred embodiment of animplanted cardiosaver processing electrogram data from implantedelectrodes, the techniques described are equally applicable to thealternate embodiment where the external cardiosaver processeselectrocardiogram data from skin surface electrodes.

In the preferred embodiment of the cardiosaver either or bothsubcutaneous electrodes or electrodes located on a pacemaker type rightventricular or atrial leads will be used. It is also envisioned that oneor more electrodes may be placed within the superior vena cava. Oneversion of the implanted cardiosaver device using subcutaneouselectrodes would have an electrode located under the skin on thepatient's left side. This could be best located between 2 and 20 inchesbelow the patient's left arm pit. The cardiosaver case that would act asthe indifferent electrode would typically be implanted like a pacemakerunder the skin on the left side of the patient's chest.

Using one or more detection algorithms, the cardiosaver can detect achange in the patient's electrogram that is indicative of a cardiacevent, such as an acute myocardial infarction, within five minutes afterit occurs and then automatically warn the patient that the event isoccurring. To provide this warning, the guardian system includes aninternal alarm sub-system (internal alarm means) within the cardiosaverand/or an external alarm system (external alarm means). In thepreferred, implanted embodiment, the cardiosaver communicates with theexternal alarm system using a wireless radio-frequency (RF) signal.

The internal alarm means generates an internal alarm signal to warn thepatient. The internal alarm signal may be a mechanical vibration, asound or a subcutaneous electrical tickle. The external alarm system(external alarm means) will generate an external alarm signal to warnthe patient. The external alarm signal is typically a sound that can beused alone or in combination with the internal alarm signal. Theinternal or external alarm signals would be used to alert the patient toat least two different types of conditions (i.e. levels of severity): an“EMERGENCY ALARM” signaling the detection of a major cardiac event (e.g.a heart attack) and the need for immediate medical attention, and a lesscritical “SEE DOCTOR ALERT” (or alarm) signaling the detection of a lessserious non life threatening condition such as exercise inducedischemia. The SEE DOCTOR alert signal would be used to tell the patientthat he is not in immediate danger but should arrange an appointmentwith his doctor in the near future. In addition to the signaling of lesscritical cardiac events, the SEE DOCTOR alert signal could also signalthe patient when the cardiosaver battery is getting low.

In the preferred embodiment, the internal EMERGENCY alarm signal wouldbe applied periodically, for example, with three pulses every 5 secondsafter the detection of a major cardiac event. It is also envisioned thatthe less critical SEE DOCTOR alert, would be signaled in a differentway, such as one pulse every 7 seconds.

The external alarm system is a hand-held portable device that mayinclude any or all of the following features:

1. an external alarm means to generate an external alarm signal to alertthe patient.

2. the capability to receive cardiac event alarms, recorded electrogramand other data from the cardiosaver

3. the capability to transmit the cardiac event alarm, recordedelectrogram and other data collected by the cardiosaver to a medicalpractitioner at a remote location.

4. an “alarm-off” or disable button that when depressed can acknowledgethat the patient is aware of the alarm and will turn off internal andexternal alarm signals.

5. a display (typically an LCD panel) to provide information and/orinstructions to the patient by a text message and the display ofsegments of the patient's electrogram.

6. the ability to provide messages including instructions to the patientvia a pre-recorded human voice.

7. a patient initiated electrogram capture initiated by a “Panic Button”to allow the patient, even when there has been no alarm, to initiatetransmission of electrogram data from the cardiosaver to the externalalarm system for transmission to a medical practitioner.

8. a patient initiated electrogram capture to initiate transmission ofelectrogram data from the cardiosaver to the external alarm system fordisplay to a medical practitioner using the display on the externalalarm system.

9. the capability to automatically turn the internal and external alarmsoff after a reasonable (initial alarm-on) period that is typically lessthan 30 minutes if the alarm-off button is not used. This feature mightalso be implemented within the cardiosaver implant.

If the alarm disable button is not used by the patient to indicateacknowledgement of awareness of an EMERGENCY alarm, it is envisionedthat instead of completely stopping all alarm signals to the patientafter the first period of time which is an initial alarm-on period, areminder alarm signal would be turned on for a second time period whichis a reminder alarm on-period of time that would follow an off-period oftime during which time the alarm signal is turned off.

The reminder alarm signal might be repeated periodically for a thirdlonger time period which is a periodic reminder time period. Each of therepeated reminder alarm signals would last for the reminder alarmon-period and would be followed by an alarm off-period. The periodicreminder time period would typically be 3 to 5 hours because after threeto five hours the patient's advantage in being alerted to seek medicalattention for a severe cardiac event like an AMI is mostly lost. Thealarm off-period between the periodic reminder alarm signals couldeither remain constant, increase or decrease over the periodic remindertime period. For example, after an initial alarm-on time period of fiveminutes a 30 second long reminder alarm signal might occur every 10minutes for a periodic reminder time period of 3 hours, (i.e. thereminder alarm on-period is 30 seconds and the alarm off-period is 9minutes and 30 seconds). It is also envisioned that the alarm off-periodmight change during the periodic reminder time period. For example, theoff-period in the first hour of the periodic reminder time period mightbe 10 minutes increasing to 20 minutes in the last hour of the periodicreminder time period.

Text and/or spoken instructions may include a message that the patientshould promptly take some predetermined medication such as chewing anaspirin, placing a nitroglycerine tablet under his tongue, inhaling ornasal spraying a single or multiple drug combination and/or injectingthrombolytic drugs into a subcutaneous drug port. The messagingdisplayed by or spoken from the external alarm system and/or a phonecall from a medical practitioner who receives the alarm could alsoinform the patient that he should wait for the arrival of emergencymedical services or he should promptly proceed to an emergency medicalfacility. It is envisioned that the external alarm system can havedirect connection to a telephone line and/or work through cell phone orother wireless networks.

If a patient seeks care in an emergency room, the external alarm systemcould provide a display to the medical practitioners in the emergencyroom of both the electrogram segment that caused the alarm and thebaseline electrogram segment against which the electrogram that causedthe alarm was compared. The ability to display both baseline and alarmelectrogram segments will significantly improve the ability of theemergency room physician to properly identify AMI.

A preferred embodiment of the external alarm system consists of anexternal alarm transceiver and a handheld computer. The external alarmtransceiver having a standardized interface, such as Compact Flashadapter interface, a secure digital (SD) card interface, a multi-mediacard interface, a memory stick interface or a PCMCIA card interface. Thestandardized interface will allow the external alarm transceiver toconnect into a similar standardized interface slot that is present inmany handheld computers such as a Palm Pilot or Pocket PC. An advantageof this embodiment is that the handheld computer can cost effectivelysupply the capability for text and graphics display and for playingspoken messages.

Using a handheld computer, such as the Thera™ by Audiovox™ that combinesa Pocket PC with having an SD/Multimedia interface slot with a cellphone having wireless internet access, is a solution that can easily beprogrammed to provide communication between the external alarm systemand a diagnostic center staffed with medical practitioners.

The panic button feature, which allows a patient-initiated electrogramcapture and transmission to a medical practitioner, will provide thepatient with a sense of security knowing that, if he detects symptoms ofa heart-related ailment such as left arm pain, chest pain orpalpitations, he can get a fast review of his electrogram. Such a reviewwould allow the diagnosis of arrhythmias, such as premature atrial orventricular beats, atrial fibrillation, atrial flutter or other heartrhythm irregularities. The medical practitioner could then advise thepatient what action, if any, should be taken. The guardian system wouldalso be programmed to send an alarm in the case of ventricularfibrillation so that a caretaker of the patient could be informed toimmediately provide a defibrillation electrical stimulus. This ispractical as home defibrillation units are now commercially available.It is also possible that, in patients prone to ventricular fibrillationfollowing a myocardial infarction, such a home defibrillator could beplaced on the patient's chest to allow rapid defibrillation shouldventricular fibrillation occur while waiting for the emergency medicalservices to arrive.

The physician's programmer provides the patient's doctor with thecapability to set cardiosaver cardiac event detection parameters. Theprogrammer communicates with the cardiosaver using the wirelesscommunication capability that also allows the external alarm system tocommunicate with the cardiosaver. The programmer can also be used toupload and review electrogram data captured by the cardiosaver includingelectrogram segments captured before, during and after a cardiac event.

An extremely important capability of the present invention is the use ofa continuously adapting cardiac event detection program that comparesextracted features from a recently captured electrogram segment with thesame features extracted from a baseline electrogram segment at apredetermined time in the past. For example, the thresholds fordetecting an excessive ST shift would be appropriately adjusted toaccount for slow changes in electrode sensitivity or ST segment voltagelevels over time. It may also be desirable to choose the predeterminedtime in the past for comparison to take into account daily cycles in thepatient's heart electrical signals. Thus, a preferred embodiment of thepresent invention would use a baseline for comparison that is collectedapproximately 24 hours prior to the electrogram segment being examined.Such a system would adapt to both minor (benign) slow changes in thepatient's baseline electrogram as well as any daily cycle.

Use of a system that adapts to slowly changing baseline conditions is ofgreat importance in the time following the implantation of electrodeleads in the heart. This is because there can be a significant “injurycurrent” present just after implantation of an electrode and for a timeof up to a month, as the implanted electrode heals into the wall of theheart. Such an injury current may produce a depressed ST segment thatdeviates from a normal isoelectric electrogram where the PQ and STsegments are at approximately the same voltage. Although the ST segmentmay be depressed due to this injury current, the occurrence of an acutemyocardial infarction can still be detected since an acute myocardialinfarction will still cause a significant shift from this “injurycurrent” ST baseline electrogram. Alternately, the present inventionmight be implanted and the detector could be turned on after healing ofthe electrodes into the wall of the heart. This healing would be notedin most cases by the evolution to an isoelectric electrogram (i.e., PQand ST segments with approximately the same voltages).

The present invention's ST detection technique involves recording andprocessing baseline electrogram segments to calculate the threshold formyocardial infarction and/or ischemia detection. These baselineelectrogram segments would typically be collected, processed and storedonce an hour or with any other appropriate time interval.

A preferred embodiment of the present invention would save and process a10 second baseline electrogram segment once every hour. Every 30 secondsthe cardiosaver would save and process a 10 second long recentelectrogram segment. The cardiosaver would compare the recentelectrogram segment with the baseline electrogram segment fromapproximately 24 hours before (i.e. 24.+−.½ hour before).

The processing of each of the hourly baseline electrogram segments wouldinvolve calculating the average electrogram signal strength as well ascalculating the average “ST deviation”. The ST deviation for a singlebeat of an electrogram segment is defined to be the difference betweenthe average ST segment voltage and the average PQ segment voltage. Theaverage ST deviation of the baseline electrogram segment is the averageof the ST deviation of multiple (at least two) beats within the baselineelectrogram segment.

The following detailed description of the drawings fully describes howthe ST and PQ segments are measured and averaged.

An important aspect of the present invention is the capability to adjustthe location in time and duration of the ST and PQ segments used for thecalculation of ST shifts. The present invention is initially programmedwith the time interval between peak of the R wave of a beat and thestart of the PQ and ST segments of that beat set for the patient'snormal heart rate. As the patient's heart rate changes during dailyactivities, the present invention will adjust these time intervals foreach beat proportional to the R-R interval for that beat. In otherwords, if the R-R interval shortens (higher heart rate) then the ST andPQ segments would move closer to the R wave peak and would becomeshorter. ST and PQ segments of a beat within an electrogram segment aredefined herein as sub-segments of the electrogram segment. Specifically,the time interval between the R wave and the start of the ST and PQsegments may be adjusted in proportion to the R-R interval oralternately by the square root of the R-R interval. It is preferable inall cases to base these times on the R-R interval from the beat beforethe current beat. As calculating the square root is a processorintensive calculation, the preferred implementation of this feature isbest done by pre-calculating the values for the start of PQ and STsegments during programming and loading these times into a simple lookuptable where for each R-R interval, the start times and/or durations forthe segments is stored.

It is envisioned that a combination of linear and square root techniquescould be used where both the time interval between the R wave and thestart of the ST segment (T.sub.ST) and the duration of the ST segment(D.sub.ST) are proportional to the square root of the R-R interval,while the time interval between the R wave and the start of the PQsegment (T.sub.PQ) and the duration of the PQ segment (D.sub.PQ) arelinearly proportional to the R-R interval.

It is also envisioned that the patient would undergo a stress testfollowing implant, the electrogram data collected would be transmittedto the physician's programmer and the parameters T.sub.ST, D.sub.ST,T.sub.PQ and D.sub.PQ would be automatically selected by the programmerbased on the electrogram data from the stress test. The data from thestress test would cover each of the heart rate ranges and could also beused by the programmer to generate excessive ST shift detectionthresholds for each of the heart rate ranges. In each heart rate rangeof the implant the detection threshold would typically be set based onthe mean and standard deviation of the ST shifts seen during the stresstest. For example, one could set the detection threshold for each heartrate range to the value of the mean ST shift plus or minus a multiple(e.g. three) times the standard deviation. In each case where theprogrammer can automatically select parameters for the ST shiftdetection algorithm, a manual override would also be available to themedical practitioner. Such an override is of particular importance as itallows adjustment of the algorithm parameters to compensate for missedevents or false positive detections.

The difference between the ST deviation on any single beat in a recentlycollected electrogram segment and a baseline average ST deviationextracted from a baseline electrogram segment is defined herein as the“ST shift” for that beat. The present invention envisions that detectionof acute myocardial infarction and/or ischemia would be based oncomparing the ST shift of one or more beats with a predetermineddetection threshold “H.sub.ST”.

In U.S. application Ser. No. 10/051,743 that is incorporated herein byreference, Fischell describes a fixed threshold for detection that isprogrammed by the patient's doctor. The present invention envisions thatthe threshold should rather be based on some percentage “P.sub.ST” ofthe average signal strength extracted from the baseline electrogramsegment where P.sub.ST is a programmable parameter of the cardiosaverdevice. The “signal strength” can be measured as peak-to-peak signalvoltage, RMS signal voltage or as some other indication of signalstrength such as the difference between the average PQ segment amplitudeand the peak R wave amplitude.

Similarly, it is envisioned that the value of P.sub.ST might be adjustedas a function of heart rate so that a higher threshold could be used ifthe heart rate is elevated, so as to not trigger on exercise that insome patients will cause minor ST segment shifts when there is not aheart attack occurring. Alternately, lower thresholds might be used withhigher heart rates to enhance sensitivity to detect exercise-inducedischemia. One embodiment of the present invention has a table stored inmemory where values of P.sub.ST for a preset number of heart rateranges, (e.g. 50-80, 81-90, 91-100, 101-120, 121-140) might be storedfor use by the cardiosaver detection algorithm in determining if anacute myocardial infarction or exercise induced ischemia is present.

Thus it is envisioned that the present invention would use the baselineelectrogram segments in 3 ways.

1. To calculate a baseline average value of a feature such as ST segmentvoltage or ST deviation that is then subtracted from the value of thesame feature in recently captured electrogram segments to calculate theshift in the value of that feature. E.g. the baseline average STdeviation is subtracted from the amplitude of the ST deviation on eachbeat in a recently captured electrogram segment to yield the ST shiftfor that beat.

2. To provide an average signal strength used in calculating thethreshold for detection of a cardiac event. This will improve detectionby compensating for slow changes in electrogram signal strength overrelatively long periods of time.

3. To provide a medical practitioner with information that willfacilitate diagnosis of the patient's condition. For example, thebaseline electrogram segment may be transmitted to a remotely locatedmedical practitioner and/or displayed directly to a medical practitionerin the emergency room.

For the purposes of the present invention, the term adaptive detectionalgorithm is hereby defined as a detection algorithm for a cardiac eventwhere at least one detection-related threshold adapts over time so as tocompensate for relatively slow (longer than an hour) changes in thepatient's normal electrogram.

The present invention might also include an accelerometer built into thecardiosaver where the accelerometer is an activity sensor used todiscriminate between elevated heart rate resulting from patient activityas compared to other causes.

It is also envisioned that the present invention could have specificprogramming to identify a very low heart rate (bradycardia) or a veryhigh heart rate (tachycardia or fibrillation). While a very low heartrate is usually not of immediate danger to the patient, its persistencecould indicate the need for a pacemaker. As a result, the presentinvention could use the “SEE DOCTOR” alert along with an optionalmessage sent to the external alarm system to alert the patient that hisheart rate is too low and that he should see his doctor as soon asconvenient. On the other hand, a very high heart rate can signalimmediate danger thus it would be desirable to initiate an EMERGENCY ina manner similar to that of acute myocardial infarction detection. Whatis more, detections of excessive ST shift during high heart rates may bedifficult and if the high heart rate is the result of a heart attackthen it is envisioned that the programming of the present inventionwould use a major event counter that would turn on the alarm if thedevice detects a combination of excessive ST shift and overly high heartrate.

Another early indication of acute myocardial infarction is a rapidchange in the morphology of the T wave. Unfortunately, there are manynon-AMI causes of changes in the morphology of a T wave. However, thesechanges typically occur slowly while the changes from an AMI occurrapidly. Therefore one embodiment of this invention uses detection of achange in the T wave as compared to a baseline collected a short time(less than 30 minutes) in the past. The best embodiment is probablyusing a baseline collected between 1 and 5 minutes in the past. Such a Twave detector could look at the amplitude of the peak of the T wave. Analternate embodiment of the T wave detector might look at the averagevalue of the entire T wave as compared to the baseline. The thresholdfor T wave shift detection, like that of ST shift detection, can be apercentage P.sub.T of the average signal strength of the baselineelectrogram segment. P.sub.T could differ from P.sub.ST if bothdetectors are used simultaneously by the cardiosaver.

In its simplest form, the “guardian system” includes only thecardiosaver and a physician's programmer. Although the cardiosaver couldfunction without an external alarm system where the internal alarmsignal stays on for a preset period of time, the external alarm systemis highly desirable. One reason it is desirable is the button on theexternal alarm system that provides the means for of turning off thealarm in either or both the implanted device (cardiosaver) and theexternal alarm system. Another very important function of the externalalarm system is to facilitate display of both the baseline and alarmelectrogram segments to a treating physician to facilitate rapiddiagnosis and treatment for the patient.

As an implantable device, the present invention cardiosaver mustconserve power to allow a reasonable lifetime in a cosmeticallyacceptable package size. In U.S. Pat. No. 6,609,023, Fischell et aldescribe how the cardiosaver collects and processes electrogram data fora first predetermined, “segment time period” (e.g. 10 seconds) to lookfor a cardiac event and then going to a lower power usage sleep statefor a second predetermined “sleep state time period” (e.g. 20 seconds).Although it is desirable to look for cardiac events every 30 seconds asdescribed by Fischell et al, it is possible to decrease the use ofelectrical power by extending the time duration of the sleep state timeperiod to be greater than 20 seconds. Extending implant lifetime bydecreasing electrical power usage can be accomplished by utilizing alonger time duration for the sleep state time period to be (for example)on the order of 50 to 80 seconds.

While a 50 to 80 sleep state time period with a 10 second time durationfor the segment time period of data collection would increase the lifeof the implant, the total cycle times of 60 to 90 seconds is acomparatively long time to wait if cardiac events are to be quicklydetected. The present invention cardiosaver utilizes an adaptive cycletime where the sleep state time period following detection of an“abnormal” electrogram segment is shorter than the sleep state timeperiod following detection of an electrogram segment that has nodetected abnormality. For example, the sleep state time period could be80 seconds following an electrogram segment where no abnormality isdetected and 20 seconds following an electrogram segment where anyabnormality (e.g. excessive ST shift or arrhythmia) is detected. In thisway, the function during any irregularity of heart signal would be thesame as the Fischell et al. cardiosaver, yet significant power savingswould be created during normal functioning of the heart.

It is also envisioned that the sleep state time period could be evenmore adaptive so that the length of the sleep state time might berelated to the number of successive normal (no abnormality detected)electrogram segments. For example, one normal segment would be followedby a sleep state time period of 40 seconds, two normal segments by 50seconds, 3 normal segments by 80 seconds, and 4 or more normal segmentsby 110 seconds. There would typically be a maximum sleep state timeperiod used during long periods when all electrogram segments are normaland a minimum sleep time period that would be used following anydetected abnormality. The maximum and minimum sleep times could bepreset or programmable.

An abnormal electrogram segment is an electrogram segment where one ormore heart signal parameters extracted during the processing of theelectrogram segment by the cardiosaver meets the criteria for anabnormal electrogram segment. The criteria for an abnormal electrogramsegment can be the same criteria used for detecting a cardiac eventwithin the electrogram segment. It is also envisioned that the criteriafor detecting an abnormal electrogram segment could be less stringentthat the criteria for detecting a cardiac event. For example, anabnormal segment might be detected using a threshold lower by a presetpercentage (e.g. 50%) than the respective threshold for the indicationof a cardiac event. In this way, the time to detection of the eventmight be reduced by getting to the shorter sleep time more quickly.

It is also highly desirable for the present invention guardian system toallow real time or near real time display of electrogram data fordiagnostic purposes. Such a display could be of great value in anemergency setting where fast review of the patient's current heartsignal is important. In a real time mode, the cardiosaver 5 of FIG. 1would simultaneously collect and transmit electrogram data to theexternal equipment 7 of FIG. 1.

In the near real time mode, the cardiosaver 5 would collect anelectrogram segment, process the electrogram segment looking forabnormalities and then transmit the segment to the external equipment 7.A typical cycle time for the near real time mode would be 15 secondsincluding 10 seconds for electrogram segment collection, 1 second forprocessing and 4 seconds for transmission to the external equipment 7.The results of the processing might also be transmitted along with thesegment.

Thus it is an object of this invention is to have a cardiosaver designedto detect the occurrence of a cardiac event by comparing baselineelectrogram data from a first predetermined time with recent electrogramdata from a second predetermined time.

Another object of the present invention is to have a Guardian systemwhere the electrogram data collected during a preset period (such asduring a stress test) is used by the programmer to automatically selectdetection parameters for the ST shift detection algorithm.

Another object of the present invention is to have a Guardian systemwith at least two levels of severity of patient alarm/alerting where themore severe EMERGENCY alarm alerts the patient to seek immediate medicalattention.

Another object of the present invention is to have a cardiac eventdetected by comparing at least one heart signal parameter extracted froman electrogram segment captured at a first predetermined time by animplantable cardiosaver with the same at least one heart signalparameter extracted from an electrogram segment captured at a secondpredetermined time.

Another object of the present invention is to have acute myocardialinfarction detected by comparing recent electrogram data to baselineelectrogram data from the same time of day (i.e. approximately 24 hoursin the past).

Another object of the present invention is to have acute myocardialinfarction detected by comparing the ST deviation of the beats in arecently collected electrogram segment to the average ST deviation oftwo or more beats of a baseline electrogram segment. Another object ofthe present invention is to have acute myocardial infarction detected bycomparing the ST segment voltage of the beats in a recently collectedelectrogram segment to the average ST segment voltage of two or morebeats of a baseline electrogram segment.

Another object of the present invention is to have the threshold(s) fordetecting the occurrence of a cardiac event adjusted by a cardiosaverdevice to compensate for slow changes in the average signal level of thepatient's electrogram.

Another object of the present invention is to have the threshold fordetection of a cardiac event adjusted by a cardiosaver device tocompensate for daily cyclic changes in the average signal level of thepatient's electrogram.

Another object of the present invention is to have an external alarmsystem including an alarm off button that will turn off either or bothinternal and external alarm signals initiated by an implantedcardiosaver.

Another object of the present invention is to have the alarm signalgenerated by a cardiosaver automatically turn off after a preset periodof time.

Still another object of this invention is to use the cardiosaver to warnthe patient that an acute myocardial infarction has occurred by means ofa subcutaneous vibration.

Still another object of this invention is to have the cardiac eventdetection require that at least a majority of the beats exhibit anexcessive ST shift before identifying an acute myocardial infarction.

Still another object of this invention is to have the cardiac eventdetection require that excessive ST shift still be present in at leasttwo electrogram segments separated by a preset period of time.

Still another object of this invention is to have the cardiac eventdetection require that excessive ST shift still be present in at leastthree electrogram segments separated by preset periods of time.

Yet another object of the present invention is to have a threshold fordetection of excessive ST shift that is dependent upon the averagesignal strength calculated from a baseline electrogram segment.

Yet another object of the present invention is to have a threshold fordetection of excessive ST shift that is a function of the differencebetween the average PQ segment amplitude and the R wave peak amplitudeof a baseline electrogram segment.

Yet another object of the present invention is to have a threshold fordetection of excessive ST shift that is a function of the averageminimum to maximum (peak-to-peak) voltage for at least two beatscalculated from a baseline electrogram segment.

Yet another object of the present invention is to have the ability todetect a cardiac event by the shift in the amplitude of the T wave of anelectrogram segment at a second predetermined time as compared with theaverage baseline T wave amplitude from a baseline electrogram segment ata first predetermined time.

Yet another object of the present invention is to have the ability todetect a cardiac event by the shift in the T wave deviation of at leastone beat of an electrogram segment at a second predetermined time ascompared with the average baseline T wave deviation from an electrogramsegment at a first predetermined time.

Yet another object of the present invention is to have the first andsecond predetermined times for T wave amplitude and/or deviationcomparison be separated by less than 30 minutes.

Yet another object of the present invention is to have the baselineelectrogram segment used for ST segment shift detection and the baselineelectrogram segment used for T wave shift detection be collected atdifferent times.

Yet another object of the present invention is to have an initialalarm-on patient alerting period followed by a reminder alarm thatperiodically cycles on and off over a periodic reminder alarm period.

Yet another object of the present invention is to have an individualized(patient specific) “normal” heart rate range such that the upper andlower limits of “normal” are programmable using the cardiosaverprogrammer.

Yet another object of the present invention is to have one or moreindividualized (patient specific) “elevated” heart rate ranges such thatthe upper and lower limits of each “elevated” range are programmableusing the cardiosaver programmer.

Yet another object of the present invention is to allow the thresholdfor detection of an excessive ST shift be different for the “normal”heart rate range as compared to one or more “elevated” heart rateranges.

Yet another object of the present invention is to allow real time ornear real time display of electrogram data for diagnostic purposes.

Yet another object of the present invention is to have the time periodbetween collections of electrogram data vary, where the time period islengthened when the electrogram is normal and shortened when theelectrogram is abnormal.

Yet another object of the present invention is to have differentcriteria for the normal/abnormal electrogram decision that influencesthe time period between collections of electrogram data as compared withthe criteria for detecting a cardiac event.

These and other objects and advantages of this invention will becomeobvious to a person of ordinary skill in this art upon reading of thedetailed description of this invention including the associated drawingsas presented herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a guardian system for the detection of a cardiacevent and for warning the patient that a cardiac event is occurring.

FIG. 2 illustrates a normal electrogram pattern and also shows asuperimposed elevated ST segment that would be indicative of an acutemyocardial infarction.

FIG. 3 is a plan view of the cardiosaver showing the cardiosaverelectronics module and two electrical leads each having one electrode.

FIG. 4 is a block diagram of the cardiosaver.

FIG. 5 is a block diagram of the cardiosaver event detection program.

FIG. 6 illustrates the extracted electrogram segment features used tocalculate ST shift.

FIG. 7 is a block diagram of the baseline parameter extractionsubroutine of the cardiosaver event detection program.

FIG. 8 is a block diagram of the alarm subroutine of the cardiosaverevent detection program.

FIG. 9 is a block diagram of the hi/low heart rate subroutine of thecardiosaver event detection program.

FIG. 10 is a block diagram of the ischemia subroutine of the cardiosaverevent detection program

FIG. 11 is a diagram of the conditions that trigger cardiosaver alarms.

FIG. 12 is a block diagram of the unsteady heart rate subroutine of thecardiosaver event detection program.

FIG. 13 is an alternate embodiment of the guardian system.

FIG. 14 illustrates the preferred physical embodiment of the externalalarm transceiver.

FIG. 15 illustrates the physical embodiment of the combined externalalarm transceiver and pocket PC, and FIG. 16 is a block diagram of anembodiment of an external alarm transceiver.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates one embodiment of the guardian system 10 consistingof an implanted cardiosaver 5 and external equipment 7. The batterypowered cardiosaver 5 contains electronic circuitry that can detect acardiac event such as an acute myocardial infarction or arrhythmia andwarn the patient when the event occurs. The cardiosaver 5 can store thepatient's electrogram for later readout and can send wireless signals 53to and receive wireless signals 54 from the external equipment 7. Thefunctioning of the cardiosaver 5 will be explained in greater detailwith the assistance of FIG. 4.

The cardiosaver 5 has two leads 12 and 15 that have multi-wireelectrical conductors with surrounding insulation. The lead 12 is shownwith two electrodes 13 and 14. The lead 15 has subcutaneous electrodes16 and 17. In fact, the cardiosaver 5 could utilize as few as one leador as many as three and each lead could have as few as one electrode oras many as eight electrodes. Furthermore, electrodes 8 and 9 could beplaced on the outer surface of the cardiosaver 5 without any wires beingplaced externally to the cardiosaver 5.

The lead 12 in FIG. 1 could advantageously be placed through thepatient's vascular system with the electrode 14 being placed into theapex of the right ventricle. The lead 12 with electrode 13 could beplaced in the right ventricle or right atrium or the superior vena cavasimilar to the placement of leads for pacemakers and ImplantableCoronary Defibrillators (ICDs). The metal case 11 of the cardiosaver 5could serve as an indifferent electrode with either or both electrodes13 and/or 14 being active electrodes. It is also conceived that theelectrodes 13 and 14 could be used as bipolar electrodes. Alternately,the lead 12 in FIG. 1 could advantageously be placed through thepatient's vascular system with the electrode 14 being placed into theapex of the left ventricle. The electrode 13 could be placed in the leftatrium.

The lead 15 could advantageously be placed subcutaneously at anylocation where the electrodes 16 and/or 17 would provide a goodelectrogram signal indicative of the electrical activity of the heart.Again for this lead 15, the case 11 of the cardiosaver 5 could be anindifferent electrode and the electrodes 16 and/or 17 could be activeelectrodes or electrodes 16 and 17 could function together as bipolarelectrodes. The cardiosaver 5 could operate with only one lead and asfew as one active electrode with the case of the cardiosaver 5 being anindifferent electrode. The guardian system 10 described herein canreadily operate with only two electrodes.

One embodiment of the cardiosaver device 5 using subcutaneous lead 15would have the electrode 17 located under the skin on the patient's leftside. This could be best located between 2 and 20 inches below thepatient's left arm pit. The cardiosaver case 11 could act as theindifferent electrode and would typically be implanted under the skin onthe left side of the patient's chest.

FIG. 1 also shows the external equipment 7 that consists of aphysician's programmer 68 having an antenna 70, an external alarm system60 including a charger 166. The external equipment 7 provides means tointeract with the cardiosaver 5. These interactions include programmingthe cardiosaver 5, retrieving data collected by the cardiosaver 5 andhandling alarms generated by the cardiosaver 5.

The purpose of the physician's programmer 68 shown in FIG. 1 is to setand/or change the operating parameters of the implantable cardiosaver 5and to read out data stored in the memory of the cardiosaver 5 such asstored electrogram segments. This would be accomplished by transmissionof a wireless signal 54 from the programmer 68 to the cardiosaver 5 andreceiving of telemetry by the wireless signal 53 from the cardiosaver 5to the programmer 68. When a laptop computer is used as the physician'sprogrammer 68, it would require connection to a wireless transceiver forcommunicating with the cardiosaver 5. Such a transceiver could beconnected via a standard interface such as a USB, serial or parallelport or it could be inserted into the laptop's PCMCIA card slot. Thescreen on the laptop would be used to provide guidance to the physicianin communicating with the cardiosaver 5. Also, the screen could be usedto display both real time and stored electrograms that are read out fromthe cardiosaver 5.

In FIG. 1, the external alarm system 60 has a patient operated initiator55, an alarm disable button 59, a panic button 52, an alarm transceiver56, an alarm speaker 57 and an antenna 161 and can communicate withemergency medical services 67 with the modem 165 via the communicationlink 65.

If a cardiac event is detected by the cardiosaver 5, an alarm message issent by a wireless signal 53 to the alarm transceiver 56 via the antenna161. When the alarm is received by the alarm transceiver 56 a signal 58is sent to the loudspeaker 57. The signal 58 will cause the loudspeakerto emit an external alarm signal 51 to warn the patient that an eventhas occurred. Examples of external alarm signals 51 include a periodicbuzzing, a sequence of tones and/or a speech message that instructs thepatient as to what actions should be taken. Furthermore, the alarmtransceiver 56 can, depending upon the nature of the signal 53, send anoutgoing signal over the link 65 to contact emergency medical services67. When the detection of an acute myocardial infarction is the cause ofthe alarm, the alarm transceiver 56 could automatically notify emergencymedical services 67 that a heart attack has occurred and an ambulancecould be sent to treat the patient and to bring him to a hospitalemergency room.

If the remote communication with emergency medical services 67 isenabled and a cardiac event alarm is sent within the signal 53, themodem 165 will establish the data communications link 65 over which amessage will be transmitted to the emergency medical services 67. Themessage sent over the link 65 may include any or all of the followinginformation: (1) a specific patient is having an acute myocardialinfarction or other cardiac event, (2) the patient's name, address and abrief medical history, (3) a map and/or directions to where the patientis located, (4) the patient's stored electrogram including baselineelectrogram data and the specific electrogram segment that generated thealarm (5) continuous real time electrogram data, and (6) a prescriptionwritten by the patient's personal physician as to the type and amount ofdrug to be administered to the patient in the event of a heart attack.If the emergency medical services 67 includes an emergency room at ahospital, information can be transmitted that the patient has had acardiac event and should be on his way to the emergency room. In thismanner the medical practitioners at the emergency room could be preparedfor the patient's arrival.

The communications link 65 can be either a wired or wireless telephoneconnection that allows the alarm transceiver 56 to call out to emergencymedical services 67. The typical external alarm system 60 might be builtinto a Pocket PC or Palm Pilot PDA where the alarm transceiver 56 andmodem 165 are built into insertable cards having a standardizedinterface such as compact flash cards, PCMCIA cards, multimedia, memorystick or secure digital (SD) cards. The modem 165 can be a wirelessmodem such as the Sierra AirCard 300 or the modem 165 may be a wiredmodem that connects to a standard telephone line. The modem 165 can alsobe integrated into the alarm transceiver 56.

The purpose of the patient operated initiator 55 is to give the patientthe capability for initiating transmission of the most recently capturedelectrogram segment from the cardiosaver 5 to the external alarm system60. This will enable the electrogram segment to be displayed for amedical practitioner.

Once an internal and/or external alarm signal has been initiated,depressing the alarm disable button 59 will acknowledge the patient'sawareness of the alarm and turn off the internal alarm signal generatedwithin the cardiosaver 5 and/or the external alarm signal 51 playedthrough the speaker 57. If the alarm disable button 59 is not used bythe patient to indicate acknowledgement of awareness of a SEE DOCTORalert or an EMERGENCY alarm, it is envisioned that the internal and/orexternal alarm signals would stop after a first time period (an initialalarm-on period) that would be programmable through the programmer 68.

For EMERGENCY alarms, to help prevent a patient ignoring or sleepingthrough the alarm signals generated during the initial alarm-on period,a reminder alarm signal might be turned on periodically during afollow-on periodic reminder time period. This periodic reminder time istypically much longer than the initial alarm-on period. The periodicreminder time period would typically be 3 to 5 hours because after 3 to5 hours the patient's advantage in being alerted to seek medicalattention for a severe cardiac event like an AMI is mostly lost. It isalso envisioned that the periodic reminder time period could also beprogrammable through the programmer 68 to be as short as 5 minutes oreven continue indefinitely until the patient acknowledges the alarmsignal with the button 59 or the programmer 68 is used to interact withthe cardiosaver 5.

Following the initial alarm on-period there would be an alarm off-periodfollowed by a reminder alarm on-period followed by an alarm off-periodfollowed by another reminder alarm on-period and so on periodicallyrepeating until the end of the periodic reminder time period.

The alarm off-period time interval between the periodic reminders mightalso increase over the reminder alarm on-period. For example, theinitial alarm-on period might be 5 minutes and for the first hourfollowing the initial alarm-on period, a reminder signal might beactivated for 30 seconds every 5 minutes. For the second hour thereminder alarm signal might be activated for 20 seconds every 10 minutesand for the remaining hours of the periodic reminder on-period thereminder alarm signal might be activated for 30 seconds every 15minutes.

The patient might press the panic button 52 in the event that thepatient feels that he is experiencing a cardiac event. The panic button52 will initiate the transmission from the cardiosaver 5 to the externalalarm system 60 via the wireless signal 53 of both recent and baselineelectrogram segments. The external alarm system 60 will then retransmitthese data via the link 65 to emergency medical services 67 where amedical practitioner will view the electrogram data. The remote medicalpractitioner could then analyze the electrogram data and call thepatient back to offer advice as to whether this is an emergencysituation or the situation could be routinely handled by the patient'spersonal physician at some later time.

It is envisioned that there may be preset limits within the externalalarm system 60 that prevent the patient operated initiator 55 and/orpanic button from being used more than a certain number of times a dayto prevent the patient from running down the batteries in thecardiosaver 5 and external alarm system 60 as wireless transmissiontakes a relatively large amount of power as compared with otherfunctional operation of these devices.

FIG. 2 illustrates a typical electrogram signal having beats 1 and 2from some pair of implanted electrodes such as the electrode 14 and thecase 11 of FIG. 3 overlaid with an electrogram having an elevated STsegment 4 (dashed line). The various portions of the electrogram areshown as the P, Q, R, S, and T waves. These are all shown as portions ofa solid line in FIG. 2. The normal ST segment 3 of beat 2 is also shownin FIG. 2. The R-R interval 5 for beat 2 is shown as the time betweenthe R waves of beat 2 and the beat before it (beat 1).

When an acute myocardial infarction occurs, there is typically anelevation (or depression) of the ST segment 4 as shown by the dashedline in FIG. 2. It is this shift of the ST segment 4 as compared to thebaseline ST segment 3 that is a clear indicator that an acute myocardialinfarction has occurred in a significant portion of the patient'smyocardium.

Although an elevated ST segment 4 can be a good indicator of an acutemyocardial infarction, other indicators such as a sudden change of heartrate or heart wall motion, intra-coronary blood pressure or a suddendecrease in blood P0.sub.2 could also be used as independent sensingmeans or those signals could be used in addition to the voltage shift ofthe ST segment 4.

It is important to note that the electrogram from implanted electrodesmay provide a faster detection of an ST segment shift as compared to anelectrocardiogram signal obtained from skin surface electrodes. Thus theelectrogram from implanted electrodes as described herein is thepreferred embodiment of the present invention.

It is also well known that the T wave can shift very quickly when aheart attack occurs. It is envisioned that the present invention mightdetect this T wave shift as compared to a time of 1 to 5 minutes in thepast.

It is anticipated that when a patient who has a stenosis in a coronaryartery is performing a comparatively strenuous exercise his heart rateincreases and he can develop exercise induced ischemia that will alsoresult in a shift of the ST segment of his electrogram. This isparticularly true for patients who have undergone balloon angioplastywith or without stent implantation. Such patients will be informed bytheir own physician that, if their cardiosaver 5 of FIG. 1 activates analarm during exercise, that it may be indicative of the progression ofan arterial stenosis in one of the heart's arteries. Such a patientwould be advised to stop all exertion immediately and if the alarmsignal goes away as his heart rate slows, the patient should see hisdoctor as soon as convenient. If the alarm signal does not go away asthe patient's heart rate slows down into the normal range then thecardiosaver will change the alarm signal to indicate that the patientshould immediately seek medical care. As previously described, thecardiosaver 5 could emit a different signal if there is a heart attackas compared to the signal that would be produced if there were ischemiaresulting from exercise.

It is also envisioned that heart rate and the rate of change of heartrate experienced during an ST segment voltage shift can be used toindicate which alarm should be produced by the cardiosaver 5.Specifically, an ST segment shift at a near normal heart rate wouldindicate an acute myocardial infarction. An ST segment shift when thereis an elevated heart rate (e.g., greater than 100 bpm) would generallybe indicative of a progressing stenosis in a coronary artery. In anycase, if a sufficient ST segment shift occurs that results in an alarmfrom the cardiosaver 5, the patient should promptly seek medical care todetermine the cause of the alarm.

It should be understood that, depending on a patient's medicalcondition, a vigorous exercise might be as energetic as running a longdistance or merely going up a flight of stairs. After the cardiosaver 5is implanted in a patient who has undergone a stent implant, he shouldhave a stress test to determine his level of ST segment shift that isassociated with the highest level of exercise that he can attain. Thepatient's heart rate should then be noted and the cardiosaver thresholdsfor detection, described with FIGS. 5 through 9, should be programmed soas to not alarm at ST segment shifts observed during exercise. Then ifat a later time the patient experiences an increased shift of his STsegment at that pre-determined heart rate or within a heart rate range,then an alarm indicating ischemia can be programmed to occur. Theoccurrence of such an alarm can indicate that there is a progression inthe narrowing of some coronary artery that may require angiography todetermine if angioplasty, possibly including stent implantation, isrequired.

The alarm signal associated with an excessive ST shift caused by anacute myocardial infarction can be quite different from the “SEE DOCTOR”alarm means associated with progressing ischemia during exercise. Forexample, the SEE DOCTOR alert signal might be an audio signal thatoccurs once every 5 to 10 seconds. A different alarm signal, for examplean audio signal that is three buzzes every 3 to 5 seconds, may be usedto indicate a major cardiac event such as an acute myocardialinfarction. Similar alarm signal timing would typically be used for bothinternal alarm signals generated by the alarm sub-system 48 of FIG. 4and external alarm signals generated by the external alarm system 60.

In any case, a patient can be taught to recognize which signal occursfor these different circumstances so that he can take immediate responseif an acute myocardial infarction is indicated but can take anon-emergency response if progression of the narrowing of a stenosis orsome other less critical condition is indicated. It should be understoodthat other distinctly different audio alarm patterns could be used fordifferent arrhythmias such as atrial fibrillation, atrial flutter,PVC's, PAC's, etc. A capability of the physician's programmer 68 of FIG.1 would be to program different alarm signal patterns, enable or disabledetection and/or generation of associated alarm signals in thecardiosaver for any one or more of these various cardiac events. Also,the intensity of the audio alarm, vibration or electrical tickle alarmcould be adjusted to suit the needs of different patients. In order tofamiliarize the patient with the different alarm signals, the programmer68 of the present invention would have the capability to turn each ofthe different alarm signals on and off.

FIG. 3 is a plan view of the cardiosaver 5 having a case 11 and aplastic header 20. The case 11 contains the primary battery 22 and theelectronics module 18. This type of package is well known forpacemakers, implantable defibrillators and implantable tissuestimulators. Electrical conductors placed through the plastic header 20connect the electronics module 18 to the electrical leads 12 and 15,which have respectively electrodes 14 and 17. The on-case electrodes 8and 9 of FIG. 1 are not shown in FIG. 3. It should also be understoodthat the cardiosaver 5 can function with only two electrodes, one ofwhich could be the case 11. All the different configurations forelectrodes shown in FIGS. 1 and 3, such as the electrodes 8, 9, 13, 14,16 or the metal case 11 are shown only to indicate that there are avariety of possible electrode arrangements that can be used with thecardiosaver 5.

On the metal case 11, a conducting disc 31 mounted onto an insulatingdisc 32 can be used to provide a subcutaneous electrical tickle to warnthe patient that an acute myocardial infarction is occurring or to actas an independent electrode.

FIG. 4 is a block diagram of the cardiosaver 5 with primary battery 22and a secondary battery 24. The secondary battery 24 is typically arechargeable battery of smaller capacity but higher current or voltageoutput than the primary battery 22 and is used for short term highoutput components of the cardiosaver 5 like the RF chipset in thetelemetry sub-system 46 or the vibrator 25 attached to the alarmsub-system 48. An important feature of the present invention.cardiosaver is the dual battery configuration where the primary battery22 will charge the secondary battery 24 through the charging circuit 23.The primary battery 22 is typically a larger capacity battery than thesecondary battery 24. The primary battery also typically has a lowerself discharge rate as a percentage of its capacity than the secondarybattery 24. It is also envisioned that the secondary battery could becharged from an external induction coil by the patient or by the doctorduring a periodic check-up.

The electrodes 14 and 17 connect with wires 12 and 15 respectively tothe amplifier 36 that is also connected to the case 11 acting as anindifferent electrode. As two or more electrodes 12 and 15 are shownhere, the amplifier 36 would be a multi-channel amplifier. The amplifiedelectrogram signals 37 from the amplifier 36 are then converted todigital signals 38 by the analog-to-digital converter 41. The digitalelectrogram signals 38 are buffered in the First-In-First-Out (FIFO)memory 42. Processor means shown in FIG. 4 as the central processingunit (CPU) 44 coupled to memory means shown in FIG. 4 as the RandomAccess Memory (RAM) 47 can process the digital electrogram data 38stored the FIFO 42 according to the programming instructions stored inthe program memory 45. This programming (i.e. software) enables thecardiosaver 5 to detect the occurrence of a cardiac event such as anacute myocardial infarction.

A clock/timing sub-system 49 provides the means for timing specificactivities of the cardiosaver 5 including the absolute or relative timestamping of detected cardiac events. The clock/timing sub-system 49 canalso facilitate power savings by causing components of the cardiosaver 5to go into a low power standby mode in between times for electrogramsignal collection and processing. Such cycled power savings techniquesare often used in implantable pacemakers and defibrillators. In analternate embodiment, the clock/timing sub-system can be provided by aprogram subroutine run by the central processing unit 44.

In an advanced embodiment of the present invention, the clock/timingcircuitry 49 would count for a first period (e.g. 20 seconds) then itwould enable the analog-to -digital converter 41 and FIFO 42 to beginstoring data, after a second period (e.g. 10 seconds) the timingcircuitry 49 would wake up the CPU 44 from its low power standby mode.The CPU 44 would then process the 10 seconds of data in a very shorttime (typically less than a second) and go back to low power mode. Thiswould allow an on off duty cycle of the CPU 44 which often draws themost power of less than 2 seconds per minute while actually collectingelectrogram data for 20 seconds per minute.

In a preferred embodiment of the present invention the RAM 47 includesspecific memory locations for 3 sets of electrogram segment storage.These are the recent electrogram storage 472 that would store the last 2to 10 minutes of recently recorded electrogram segments so that theelectrogram data leading in the period just before the onset of acardiac event can be reviewed at a later time by the patient's physicianusing the physician's programmer 68 of FIG. 1. For example, the recentelectrogram storage 472 might contain eight 10 second long electrogramsegments that were captured every 30 seconds over the last 4 minutes.

The baseline electrogram memory 474 would provide storage for baselineelectrogram segments collected at preset times over one or more days.For example, the baseline electrogram memory 474 might contain 24baseline electrogram segments of 10 seconds duration, one from each hourfor the last day.

The event memory 476 occupies the largest part of the RAM 47. The eventmemory 476 is not overwritten on a regular schedule as are the recentelectrogram memory 472 and baseline electrogram memory 474 but istypically maintained until read out by the patient's physician with theprogrammer 68 of FIG. 1. At the time a cardiac event like excessive STshift indicating an acute myocardial infarction is detected by the CPU44, all (or part) of the entire contents of the baseline and recentelectrogram memories 472 and 474 would typically be copied into theevent memory 476 so as to save the pre-event data for later physicianreview.

In the absence of events, the event memory 476 could be used temporarilyto extend the recent electrogram memory 472 so that more data (e.g.every 10 minutes for the last 12 hours) could be held by the cardiosaver5 of FIG. 1 to be examined by a medical practitioner at the time apatient visits. This would typically be overwritten with pre- andpost-event electrogram segments following a detected event.

An example of use of the event memory 476 would have a SEE DOCTOR alertsaving the last segment that triggered the alarm and the baseline usedby the detection algorithm in detecting the abnormality. An EMERGENCYALARM would save the sequential segments that triggered the alarm, aselection of other pre-event electrogram segments, or a selection of the24 baseline electrogram segments and post-event electrogram segments.For example, the pre-event memory would have baselines from −24 hrs,−18, −12, −6, −5, −4, −3, −2 and −1 hours, recent electrogram segments(other than the triggering segments) from −5 minutes, −10, −20, −35, and−50 minutes, and post-event electrogram segments for every 5 minutes forthe 2 hours following the event and for every 15 minutes after 2 hourspost-event. These settings could be pre-set or programmable. The RAM 47also contains memory sections for programmable parameters 471 andcalculated baseline data 475. The programmable parameters 471 includethe upper and lower limits for the normal and elevated heart rateranges, and physician programmed parameters related to the cardiac eventdetection processes stored in the program memory 45. The calculatedbaseline data 475 contain detection parameters extracted from thebaseline electrogram segments stored in the baseline electrogram memory474. Calculated baseline data 475 and programmable parameters 471 wouldtypically be saved to the event memory 476 following the detection of acardiac event. The RAM 47 also includes patient data 473 that mayinclude the patient's name, address, telephone number, medical history,insurance information, doctor's name, and specific prescriptions fordifferent medications to be administered by medical practitioners in theevent of different cardiac events.

It is envisioned that the cardiosaver 5 could also contain pacemakercircuitry 170 and/or defibrillator circuitry 180 similar to thecardiosaver systems described by Fischell in U.S. Pat. No. 6,240,049.

The alarm sub-system 48 contains the circuitry and transducers toproduce the internal alarm signals for the cardiosaver 5. The internalalarm signal can be a mechanical vibration, a sound or a subcutaneouselectrical tickle or shock.

The telemetry sub-system 46 with antenna 35 provides the cardiosaver 5the means for two-way wireless communication to and from the externalequipment 7 of FIG. 1. Existing radiofrequency transceiver chip setssuch as the Ash transceiver hybrids produced by RF Microdevices, Inc.can readily provide such two-way wireless communication over a range ofup to 10 meters from the patient. It is also envisioned that short rangetelemetry such as that typically used in pacemakers and defibrillatorscould also be applied to the cardiosaver 5. It is also envisioned thatstandard wireless protocols such as Bluetooth and 802.11a or 802.11bmight be used to allow communication with a wider group of peripheraldevices.

A magnet sensor 190 may be incorporated into the cardiosaver 5. Animportant use of the magnet sensor 190 is to turn on the cardiosaver Son just before programming and implantation. This would reduce wastedbattery life in the period between the times that the cardiosaver 5 ispackaged at the factory until the day it is implanted.

The cardiosaver 5 might also include an accelerometer 175. Theaccelerometer 174 together with the processor 44 is designed to monitorthe level of patient activity and identify when the patient is active.The activity measurements are sent to the processor 44. In thisembodiment the processor 44 can compare the data from the accelerometer175 to a preset threshold to discriminate between elevated heart rateresulting from patient activity as compared to other causes.

FIG. 5 illustrates in the form of a block diagram the operation of theheart signal processing program 450 for cardiac event detection by thecardiosaver 5 of FIGS. 1-4. The heart signal processing program 450 isan example of one of many such detection programs whose instructionscould reside in the program memory 45 for use by the CPU 44 of thecardiosaver 5 as shown in FIG. 4. The main section of the heart signalprocessing program 450 begins with step 451 where the event counter “k”is set to zero indicating there have been no detected events. Next, instep 452 the cardiosaver 5 is said to sleep for X seconds. The termsleep here indicates that for a period of X seconds, the cardiosaver 5would either be placed in a low power standby mode (if available) orwould otherwise simply wait for a time of X seconds before moving tostep 453. Step 453 following 452 has an electrogram segment representingY seconds of electrogram data captured into the FIFO buffer 42 of FIG.4. .sigma. a is the data sampling rate in samples per second, thus thetotal number of samples collected in step 453 is .sigma. multiplied byY. It is envisioned that X would be a time between 5 seconds and 5minutes with 20 seconds as a preferred value. Y would be between 3 and30 seconds with 10 seconds as a preferred value. .sigma. is typicallybetween 100 and 500 samples per second with 200 samples per second beinga preferred value.

After being captured, in step 454, the Y seconds of electrogram datarepresenting the most recent electrogram segment is transferred to therecent electrogram memory 472 of FIG. 4. At this time the processing andanalysis of the data begins. Throughout the remainder of this detaileddescription of the drawings, the “Y second long electrogram segment”refers to the most recently collected Y seconds of electrogram data thathave been captured and transferred to the recent electrogram memory 472by the steps 453 and 454. The term “recent electrogram segments” refersto all of the electrogram segments stored in the recent electrogrammemory 472. For example, there could be eight total 10 second longrecent electrogram segments that were captured at 30 second intervalsover a 4 minute period.

The first processing step following the collection of the Y second longelectrogram segment is step 455 that measures the intervals between theR waves in the most Y second long electrogram segment. These R-Rintervals are then used to calculate the average heart rate and R-Rinterval variation for the Y second long electrogram segment. If theaverage heart rate is below a programmed low heart rate limit rho . . .sub.low or above a programmed high heart rate limit rho . . . sub.high,it is considered “out-of-range” and a Hi/Low heart rate subroutine 420(see FIG. 9) is run to properly respond to the condition.

If the R-R interval variation within the Y second long electrogramsegment is more than a programmed limit, the hi/low heart ratesubroutine is also run. This is an important feature of the presentinvention as PVC's and unstable heart rhythms such as a bigeminal rhythmcan cause errors in an ST shift detection algorithm that is works bestwith a steady heart rhythm. One embodiment of the present inventionidentifies an unsteady heart rate by comparing the two shortest R-Rintervals and the 2 longest intervals in the Y second long electrogramsegment. If the difference between both of the two shortest R-Rintervals and the average of the two longest R-R intervals are more thana programmed percentage .alpha., an unsteady heart rate is identified.For example the programmed percentage .alpha. might be 25% so that ifthe two shortest R-R intervals are each more than 25% less than theaverage of the two longest R-R intervals, then the heart rate isunsteady. It is envisioned that if longer times Y are used forelectrogram segment collection then it might require 3 or more “short”beats to indicated an unsteady heart rate. Any beat that is not tooshort is classified by step 455 as a normal beat. .rho . . . sub.low,rho.sub.high and a are programmable parameters typically set using theprogrammer 68 during programming of the cardiosaver 5. Typical valuesfor .rho . . . sub.low and .rho . . . sub.high would be 50 and 140 beatsper minute respectively.

If the heart rate is not high, low or unsteady as checked in step 455,the heart signal processing program 450 moves to step 456 where theaverage heart rate is compared to a programmed normal range between .rho. . . sub.low and .rho . . . sub.elevated where .rho.sub.elevated is theelevated heart rate limit that defines the upper limit of the “normalrange” (e.g. 80 beats per minute). If the patient's heart rate iselevated but not out-of-range (i.e. above .rho . . . sub.high), thepatient may be exercising and the ischemia subroutine 480 allows fordifferent cardiac event detection criteria during elevated heart ratesto reduce false positive detections of acute myocardial infarction andto detect exercise induced ischemia. An example of one embodiment of theischemia subroutine 480 is illustrated in FIG. 10.

Although the above specification describes low, high and elevated heartrate limits rho . . . sub.low, .rho . . . sub.high and rho.sub.elevated,it is envisioned that instead of heart rate (i.e. beats per second) thelimits and decision making could be set in terms or R wave to R wave(R-R) interval with the low, high and elevated limits are for R-Rinterval and are expressed in seconds per beat, milliseconds per beat orsamples per beat.

If the average heart rate of the patient is within the “normal” range instep 456, then the program 450 moves to step 457 where it looks for anexcessive ST shift on M out of N beats as compared with the baselineelectrogram segment collected at a time U.+−.W minutes in the past. Ucan be any time from 1 minute to 48 hours but to allow for daily cyclesU=24 hours is a preferred embodiment. W is half the interval betweentimes when the baseline data is saved and can be any time from 10seconds to 12 hours. For a U of 24 hours, a preferred setting would haveW equal to half an hour so that the current Y second long electrogramsegment is always being compared with a baseline electrogram segmentfrom 24.+−.½ hour before. This also means that baseline electrogramsegments are saved and processed to extract detection parameters at aninterval of twice W (2W). I.e., if W is half an hour, then the baselinedata is saved and processed once an hour. M can be any number from 1 to30 and N can be any number from M to 100. An example of a typical M andN used would be 6 out of 8 beats. It is envisioned that the first of the8 beats will typically be the beat including the 2.sup.nd R wave in theY second long electrogram segment collected in steps 453 and 454.

If one is trying to detect abnormalities in 6 out of 8 beats for apositive detection, a negative detection will occur whenever 3 OK beatswithout a detected abnormality are found (so long as it is before the 6“abnormal” beats with detected abnormalities). To save processing timeand potentially extend battery life it is desirable to have steps 457and 469 of FIG. 5 simultaneously count both the number of OK beats andthe number of abnormal beats. The steps 457 and 469 will stop processingbeats when either 3 OK beats (a negative detection) or 6 abnormal beats(a positive detection) are found. Another advantage of this technique isthat even if the Y second long electrogram segments collected in steps453 and 465 have less than 6 beats but there are at least 3 OK beats,there sufficient data to declare a negative detection (i.e. nothing iswrong). As heart attacks occur rarely, this improvement will greatlyenhance the efficiency of detection algorithm. Although the exampleabove uses 3 OK vs. 6 out of 8 abnormal beats, this technique will workfor any M out of N detection scheme where N-M+1 OK beats is sufficientto declare that no event has occurred. This enhancement will work in anydevice for detecting cardiac events whether implanted within the patientor external to the patient. This technique both looking for OK andabnormal beats can be applied throughout the subroutines of the presentinvention. For example, ST shift is detected in steps 434 and 439 ofFIG. 9 and is of particular importance with a low heart rate where theremay not be M beats to process in the Y seconds. It is also applicable tothe Unsteady Heart Rate Subroutine 410 in step 418 and can reduce thenumber of times that an additional Y second electrogram segment must becollected to get sufficient data to detect the presence or absence of anevent.

The electrogram segment length Y should be programmed to be ofsufficient length such that there will be more than N beats within the Ysecond electrogram segment for heart rates at the low limit for thenormal heart rate range. If Y is too short, then the programs 450 and460 may need to also allow for the collection of additional electrogramdata as shown in FIG. 12 for the unsteady heart rate subroutine 410.

An alternate to ST shift detection in step 457 is to process just the Twave, which can change its peak or average amplitude rapidly if there isa heart attack. The T wave can, however change its amplitude slowlyunder normal conditions so a T wave shift detector would need a muchshorter time U than that of a detector using the ST segment before the Twave. If the detector is checking for such T wave shift, i.e. a voltageshift of the T wave part of the ST segment, then it may be desirable tocheck against a baseline where U is 1 to 30 minutes and W is 15 secondsto 15 minutes. For example, U=3 minutes and W=15 seconds is a preferredsetting to catch a quickly changing T wave. This would also allow use ofrecent electrogram segments stored in the recent electrogram memory ofFIG. 4 as baseline electrogram segments for T wave shift detection. Itis envisioned that the programmer 68 of FIG. 1 would allow the patient'sdoctor to program the cardiosaver 5 to use ST segment shift or T waveshift detectors by themselves, or together simultaneously. If both wereused then the programmer 68 would allow the patient's doctor to choosewhether a positive detection will result if either technique detects anevent or only if both detect an event.

If the average heart rate is in the normal range, is not unsteady andthere is no cardiac event detection in step 457, (i.e. the electrogramsignal is indicative of a “normal” heart signal for the patient), theheart signal processing program 450 checks in step 458 if it is morethan the interval of 2W minutes since the last time baseline data wascaptured. If it has been more than 2W, the baseline parameter extractionsubroutine 440 of FIG. 7 is run.

The parameters X, Y, U and W are stored with the programmable parameters471 in the RAM 47 in FIG. 4. These parameters may be permanently set atthe time of manufacturing of the cardiosaver 5 or they may be programmedthrough the programmer 68 of FIG. 1. The calculated criteria for cardiacevent detection extracted from the baseline electrogram segments storedin baseline electrogram memory 474 are stored in the calculated baselinedata memory 475 of the RAM 47.

A typical configuration of the heart signal processing program 450 usingonly an ST shift detector, would use a sleep of X=20 seconds, followedby collection of a Y=10 second long electrogram segment. If thepatient's heart rate is in a normal range of between 50 and 80 beats perminute, step 457 would check for an excessive shift of the ST segment in6 out of 8 of the beats as compared with baseline data collected 24.+−.½hour previously.

If there has been a detected excessive ST shift in M out of N beats instep 457, the ST Verification Subroutine 460 is run to be sure that thedetected event is not a transitory change in the electrogram.

The ST Verification Subroutine 460 begins with step 461 where therecently collected Y second long electrogram segment is saved to theevent memory 476 of FIG. 4 for later review by the patient's doctor.

The ST shift verification subroutine 460 then increments the eventcounter k by 1 (step 462) and then checks (step 463) if k is equal to 3(i.e. 3 events is the trigger for an alarm. If k=3 then the alarmsubroutine 490 illustrated in FIG. 8 is run, thus declaring that therehas been a positive detection of a major cardiac event. FIG. 11illustrates examples of the combinations of conditions that can lead tok=3 and the running of the alarm subroutine 490.

Although step 463 is shown checking if k=3 as the condition for runningthe alarm subroutine 490, the number of events required could be aprogrammable parameter from k=1 to k=20. Even higher possible valuesthan k=20 might be used to avoid false positive detections. With currentaverage times from onset of a heart attack to arrival at a treatmentcenter of 3 hours, a few minutes delay for a device that should enablethe patient to easily reach a treatment center within 30 minutes isvaluable if it improves the reliability of detection.

In step 463 if k is less than 3 then the ST shift verificationsubroutine 460 proceeds to sleep Z seconds in step 464 followed bycollection (step 465) and saving (step 466) to the next location in therecent electrogram memory 472 of FIG. 4 of a new Y second longelectrogram segment. Z seconds can be different from the X seconds usedin step 452 to allow the ST shift verification subroutine 460 to lookover longer (or shorter) intervals than the main program so as to bestverify the positive detection of step 457. The term sleep here has thesame connotation as in step 452. A preferred embodiment of the presentinvention uses Z=X=20 seconds.

The ST shift verification subroutine 460 then checks for heart rateout-of-range or unsteady in step 467. As described with respect to step455 above, heart rate out-of-range means that the average heart rate inthe Y second long electrogram segment is below the low heart rate limitrho . . . sub.low or above the high heart rate limit .rho . . .sub.high.

If the heart rate is out-of range or unsteady step 467 will initiate theHi/Low subroutine 420. If the heart rate is not out-of range orunsteady, then step 468 follows to check if the heart rate is normal orelevated similar to step 456 above. If the heart rate is elevated, theischemia subroutine 480 is run. The reason for checking if the heartrate has changed is that acute myocardial infarction can induce highheart rates from tachycardia or fibrillation that might mask the STshift but are in of themselves major cardiac events whose detection willincrement the event counter k.

If the heart rate is in the normal range (i.e. not elevated), then step469 checks for an excessive ST and/or T wave shift in M out of N beatsof the Y second long electrogram segment as compared with the baselinedata extracted U.+−.W minutes in the past (similar to step 457). If noexcessive ST and/or T wave shift is seen, the subroutine 460 returns tostep 458 of the heart signal processing program 450 and then eventuallyback to step 451, the start of heart signal processing program 450. Instep 451, k is set back to 0 so that only if there are cardiac eventsdetected in three (k) successive Y second long electrogram segments,will the alarm subroutine 490 be run. In a preferred embodiment of thepresent invention, steps 457 and 469 only examine M out of N “normal”beats, ignoring any beats that are too short as determined by step 455.

It is important to note, that baseline data is extracted only when theheart rate is within the normal range and there is not an excessive STor T wave shift in M out of N beats. In one embodiment of the presentinvention, this is improved further by having the baseline parameterextraction subroutine 440 only process normal beats that individually donot exhibit an excessive ST and/or T wave shift.

FIG. 6 illustrates the features of a single normal beat 500 of anelectrogram segment and a single beat 500′ of an AMI electrogram segmentthat has a significant ST segment shift as compared with the normal beat500. Such ST segment shifting occurs within minutes following theocclusion of a coronary artery during an AMI. The beats 500 and 500′show typical heart beat wave elements labeled P, Q, R, S, and T. Thedefinition of a beat such as the beat 500 is a sub-segment of anelectrogram segment containing exactly one R wave and including the Pand Q elements before the R wave and the S and T elements following theR wave.

For the purposes of detection algorithms, different sub-segments,elements and calculated values related to the beats 500 and 500′ arehereby specified. The peak of the R wave of the beat 500 occurs at thetime T.sub.R (509). The PQ segment 501 and ST segment 505 aresub-segments of the normal beat 500 and are located in time with respectto the time T.sub.R (509) as follows:

a. The PQ segment 501 has a time duration D.sub.PQ (506) and startsT.sub.PQ (502) milliseconds before the time T.sub.R (509).

b. The ST segment 505 has a time duration D.sub.ST (508) and startsT.sub.ST (502) milliseconds after the time T.sub.R (509).

The PQ segment 501′ and ST segment 505′ are sub-segments of the beat500′ and are located in time with respect to the time T′.sub.R (509′) asfollows:

c. The PQ segment 501′ has a time duration D.sub.PQ (506) and startsT.sub.PQ (502) milliseconds before the time T′.sub.R (509′).

d. The ST segment 505′ has a time duration D.sub.ST (508) and startsT.sub.ST (502) milliseconds after the time T′.sub.R (509′).

The ST segments 505 and 505′ and the PQ segments 501 and 501′ areexamples of sub-segments of the electrical signals from a patient'sheart. The R wave and T wave are also sub-segments. The dashed linesV.sub.PQ (512) and V.sub.ST (514) illustrate the average voltageamplitudes of the PQ and ST segments 501 and 505 respectively for thenormal beat 500. Similarly the dashed lines V′.sub.PQ (512′) andV′.sub.ST (514′) illustrate the average amplitudes of the PQ and STsegments 501′ and 505′ respectively for the beat 500′. The “STdeviation” .DELTA.V (510) of the normal beat 500 and the ST deviation.DELTA.V.sub.AMI (510′) of the AMI electrogram beat 500′ are defined as:.DELTA.V(510)=V.sub.ST(514)−V.sub.PQ(512).DELTA.V.sub.AMI (510′)=V′.sub.ST(514′)−V′.sub.PQ(512′)

Note that the both beats 500 and 500′ are analyzed using the same timeoffsets T.sub.PQ and T.sub.ST from the peak of the R wave and the samedurations D.sub.PQ and D.sub.ST. In this example, the beats 500 and 500′are of the same time duration (i.e. the same heart rate). The parametersT.sub.PQ, T.sub.ST, D.sub.PQ and D.sub.ST would typically be set withthe programmer 68 of FIG. 1 by the patient's doctor at the time thecardiosaver 5 is implanted so as to best match the morphology of thepatient's electrogram signal and normal heart rate. V.sub.PQ (512),V.sub.ST (514), V.sub.R (503) and .DELTA.V (510) are examples ofper-beat heart signal parameters for the beat 500.

Although it may be effective to fix the values of time offsets T.sub.PQ(502) and T.sub.ST (504) and the durations D.sub.PQ (506) and D.sub.ST(508), it is envisioned that the time offsets T.sub.PQ and T.sub.ST andthe durations D.sub.PQ and D.sub.ST could be automatically adjusted bythe cardiosaver 5 to account for changes in the patient's heart rate. Ifthe heart rate increases or decreases, as compared with the patient'snormal heart rate, it envisioned that the offsets T.sub.PQ (502) andT.sub.ST (504) and/or the durations D.sub.PQ (506) and D.sub.ST (508)could vary depending upon the R-R interval between beats or the averageR-R interval for an electrogram segment. A simple technique for doingthis would vary the offsets T.sub.PQ and T.sub.ST and the durationsD.sub.PQ and D.sub.ST in proportion to the change in R-R interval. Forexample if the patient's normal heart rate is 60 beats per minute, theR-R interval is 1 second; at 80 beats per minute the R-R interval is0.75 seconds, a 25% decrease. This could automatically produce a 25%decrease in the values of T.sub.PQ, T.sub.ST, D.sub.PQ and D.sub.ST.Alternately, the values for T.sub.PQ, T.sub.ST, D.sub.PQ and D.sub.STcould be fixed for each of up to 20 preset heart rate ranges. In eithercase, it is envisioned that after the device has been implanted, thepatient's physician would, through the programmer 68 of FIG. 1, downloadfrom the cardiosaver 5 to the programmer 68, a recent electrogramsegment from the recent electrogram memory 472. The physician would thenuse the programmer 68 to select the values of T.sub.PQ, T.sub.ST,D.sub.PQ and D.sub.ST for the heart rate in the downloaded recentelectrogram segment. The programmer 68 would then allow the physician tochoose to either manually specify the values of T.sub.PQ, T.sub.ST,D.sub.PQ and D.sub.ST for each heart rate range or have the cardiosaver5 automatically adjust the values of T.sub.PQ, T.sub.ST, D.sub.PQ andD.sub.ST based on the R-R interval for each beat of any electrogramsegment collected in the future by the cardiosaver 5. It is alsoenvisioned that only the offset times, T.sub.PQ and T.sub.ST, might beautomatically adjusted and the durations D.sub.PQ and D.sub.ST would befixed so that the average values of the ST and PQ segments V.sub.PQ(512), V.sub.ST (514), V′.sub.PQ (512′) and V′.sub.ST (514′) wouldalways use the same number of data samples for averaging.

While the simplest method of adjusting the times T.sub.PQ and T.sub.STis to adjust them in proportion to the R-R interval from the preceding Rwave to the R wave of the current beat, a preferred embodiment of thepresent invention is to adjust the times T.sub.PQ and T.sub.ST inproportion to the square root of the R-R interval from the preceding Rwave to the R wave of the current beat. It is also envisioned that acombination of linear and square root techniques could be used whereT.sub.ST and D.sub.ST are proportional to the square root of the R-Rinterval while T.sub.PQ and D.sub.PQ are linearly proportional to theR-R interval.

When used in pacemakers or combination pacemaker/ICDs it envisioned thatthe start time T.sub.ST and duration D.sub.ST of the ST segment may havedifferent values than during sinus rhythm (when the pacemaker is notpacing) as pacing the heart changes the characteristics of ischemic STshifts causing them to occur later relative to the start of the R wave.It is also envisioned, that the offset for the start of the ST segmentmay be better measured from the S Wave instead of the R wave used forsinus rhythm when the pacemaker is not pacing. The technique of usingdifferent timing parameters for start and duration when pacing can beapplied to analysis of any sub-segment of the electrogram including thesub-segment that includes the T wave peak.

Various techniques have been used to detect the R and S waves inelectrogram data. A well known technique is to look for a change inslope that exceeds a programmed threshold. Because the polarity of thewave depends on electrode placement in surface ECG, the slope thresholdis the same for both positive and negative slopes. Because the guardiansystem has the polarity in a right ventricle to implanted device fixed,the present invention envisions using different threshold values forpositive and negative slopes to better detect paced beats and/or PVCs.

The detection algorithm may need to differentiate between R, S and Twaves so as not to miscalculate the R-R interval between beats. This canbe accomplished by measurement of the width of each of the R, S, and Twaves where the R and S are always much narrower than the T wave. It isenvisioned that the present invention would discriminate R (or S) vs. Twave by the width of the wave. For example, to be a detected R wave, thewave must have a width that is within a specified range of the R wavesthat were measured within a pre-set time such as a minute in the past.In this way if the T wave spikes up during an ischemic event it will betoo wide to be considered an R wave and the detection algorithm will notbe fooled.

Another way to accomplish the same result is to use a separate high passfilter for the signal used for R wave detection where the R wavedetector high pass filter cuts has more low frequency attenuation thanthe high pass filter used for the signal analyzed for ST segmentchanges. This technique is currently used in pacemakers and ICDs for Rwave detection but can also be applied to a stand alone cardiosaverdevice for ischemia detection. Typical high pass filter settings wouldbe as follows:

For R wave detection use a high pass filter with 6 dB attenuation at 10Hz to 20 Hz.

For ST segment shift detection use a high pass filter with 6 dBattenuation at 0.1 to 0.5 Hz.

An example of a sequence of steps used to calculate the ST deviation 510for the normal beat 500 are as follows:

1. Identify the time T.sub.R (509) for the peak of the R wave for thebeat 500,

2. Calculate the time since the previous R wave and use that time tolook up or calculate the values of T.sub.PQ, T.sub.ST, D.sub.PQ andD.sub.ST.

3. Average the amplitude of the PQ segment 501 between the times(T.sub.R−T.sub.PQ) and (T.sub.R−T.sub.PQ+D.sub.PQ) to create the PQsegment average amplitude V.sub.PQ (512),

4. Average the amplitude of the ST segment 505 between the times(T.sub.R+T.sub.ST) and (T.sub.R+T.sub.ST+D.sub.ST) to create the STsegment average amplitude V.sub.ST (514),

5. Subtract V.sub.PQ (512) from V.sub.ST (514) to produce the STdeviation .DELTA.V (510) for the beat 500.

Although only one normal beat 500 is shown here, there would typicallybe multiple beats saved in the Y second long electrogram segments storedin the recent electrogram memory 472 and the baseline electrogram memory474 of FIG. 4. At preset time intervals during the day step 458 of FIG.5 will run the baseline parameter extraction subroutine 440 that willcalculate the “average baseline ST deviation” .DELTA.V.sub.BASE definedas the average of the ST deviations .DELTA.V (510) for at least twobeats of a baseline electrogram segment. Typically the ST deviation of 4to 8 beats of the baseline electrogram segment will be averaged toproduce the average baseline ST deviation .DELTA.V.sub.BASE.

For each of “i” preset times during the day (at a time interval ofapproximately 2W) an average baseline ST deviation .DELTA.V.sub.BASE(i)will be calculated and saved in the calculated baseline data memory 475for later comparison with the ST deviation .DELTA.V (510) of each beatof a recently collected electrogram. For example, in a preferredembodiment of the present invention, the average baseline ST deviation.DELTA.V.sub.BASE(i) is collected once an hour and there are be 24values of .DELTA.V.sub.BASE(i) (.DELTA.V.sub.BASE(1),.DELTA.V.sub.BASE(2) . . . DELTA.V.sub.BASE(24)) stored in thecalculated baseline data memory 475 of FIG. 4. An excessive ST shift fora single beat of a recently collected electrogram segment is thendetected when the ST deviation .DELTA.V for that beat shifts by morethan a predetermined threshold amplitude from the average baseline STdeviation .DELTA.V.sub.BASE(i) collected approximately 24 hours before.

The ST shift of a given beat is calculated by subtracting theappropriate averaged baseline ST deviation .DELTA.V.sub.BASE(i) from theST deviation .DELTA.V for that beat. Assuming the R-R interval indicatesthat the heart rate for a beat is in the normal range then an excessiveST shift for a single beat is detected if (.DELTA.V−.DELTA.V.sub.BASE(i)) is greater than the normal ST shift thresholdH.sub.normal for the normal heart rate range. The heart signalprocessing program 450 of FIG. 5 requires that such an excessive STshift be positively identified in M out of N beats in three successiverecent electrogram segments before the alarm subroutine 490 isactivated. The threshold H.sub.normal may be a fixed value that does notchange over time and is set at the time of programming of thecardiosaver 5 with the programmer 68 of FIG. 1.

In a preferred embodiment, the threshold for detection of excessive STshift is not fixed but is calculated as H.sub.ST(i) from the i′thbaseline electrogram segment stored in the baseline electrogram memory474 of FIG. 4. To do this the difference between the amplitude of thepeak of the R wave V.sub.R (503) and the average PQ segment amplitudeV.sub.PQ (512) are calculated for each of at least 2 beats of eachbaseline electrogram segment by the baseline parameter extractionsubroutine 440: The average value AR(i) of this difference(V.sub.R-V.sub.PQ) for at least two beats of the i′th baselineelectrogram segment can be used to produce a threshold for ST shiftdetection H.sub.ST(i) that is proportional to the signal strength of thei′th baseline electrogram segment. The advantage of this technique isthat, if the signal strength of the electrogram changes slowly overtime, the threshold H.sub.ST(i) for excessive ST shift detection willchange in proportion.

The preferred embodiment of the present invention would have a presetpercentage P.sub.ST that is multiplied by .DELTA.R(i) to obtain thethreshold H.sub.ST(i)=P.sub.ST X .DELTA.sub.R(i). Thus, the thresholdH.sub.ST(i) would be a fixed percentage of the average height of the Rwave peaks over the ST segments of the i′th baseline electrogramsegment. For example, if P.sub.ST is 25% an excessive ST shift on agiven beat would be detected if the ST shift(.DELTA.V−.DELTA.V.sub.BASE(i)) is greater than the thresholdH.sub.ST(i) where H.sub.ST(i) is 25% of the average PQ to R height.DELTA.R(i) of the i′th baseline electrogram segment.

In a preferred embodiment of the present invention heart signalprocessing program 450 of FIG. 5, the value X and Z are both 20 seconds,Y is 10 seconds, 2W is 60 minutes, U is 24 hours, W is 30 minutes, M is6 and N is 8. Therefore the steps 457 and 469 of FIG. 5 will check forexcessive ST shifts in 6 out of 8 beats from of the Y=10 second longelectrogram segment captured every 30 seconds as compared withparameters extracted from the baseline electrogram segment captured24.+−.½ hour before. In this preferred embodiment baseline electrogramsegments are captured once per hour.

It is also envisioned that the patient would undergo a stress testfollowing implant. The electrogram data collected by the implant 5 wouldbe transmitted to the programmer 68 of FIG. 1, and one or more of theparameters T.sub.PQ (502), T.sub.ST (504), D.sub.PQ (506) and D.sub.ST(508) of FIG. 6 would be automatically selected by the Programmer basedon the electrogram data from the stress test. The data from the stresstest should cover multiple heart rate ranges and would also be used bythe programmer 68 to generate the excessive ST shift detectionpercentage thresholds P.sub.ST for each of the heart rate ranges. Ineach case where the programmer 68 automatically selects parameters forthe ST shift detection algorithm, a manual override would also beavailable to the medical practitioner. Such an override is of particularimportance as it allows adjustment of the algorithm parameters tocompensate for missed events or false positive detections.

The S wave peak voltage V.sub.S (507) is also shown on the baseline beat500 in FIG. 6. While the preferred embodiment of the present inventionuses the average PQ to R wave amplitude .DELTA.R(i) as the normalizationvoltage for setting the threshold H.sub.ST(i), it is also envisionedthat normalization voltage could be the average of the entire R wave toS wave amplitude (V.sub.R-V.sub.S) or it could be the larger of.DELTA.R(i) or the PQ to S amplitude .DELTA.S(i)=V.sub.S-V.sub.PQ. It isimportant to note here that the threshold H.sub.ST(i) is set as apercentage of the baseline average signal amplitude. This is importantbecause the baseline signal is only collected if the electrogram isnormal and therefore the thresholds would not be affected by transientchanges in signal amplitude (e.g. R wave height) that can occur duringan ST elevation myocardial infarction. Therefore, for the purposes ofthe present invention the threshold H.sub.ST(i) is calculated as apercentage of the average signal amplitude of at least two beats of thebaseline electrogram segment where the average signal amplitude of thebaseline segment can be any of the following:

the average PQ segment to R voltage difference .DELTA.R(i),

the peak-to-peak voltage of the beat (i.e. the R to S wave voltagedifference) (V.sub.R-V.sub.S),

the average PQ segment to S wave voltage difference .DELTA.S(i),

the larger of .DELTA.R(i) or .DELTA.S(i), or

any average signal amplitude calculated from at least two beats of thebaseline electrogram segment.

FIG. 7 illustrates a preferred embodiment of the baseline extractionsubroutine 440. The subroutine 440 begins in step 439 by saving in thei′th memory location in baseline electrogram memory 474 of FIG. 4, thelast Y second long electrogram segment saved into the “Recent”electrogram memory in step 454 of FIG. 5. This Y seconds of electrogramdata then becomes the baseline electrogram segment for calculatingparameters for detection to be used during the 2 W long period of timeU.+−.W minutes in the future.

Next in step 441 the baseline extraction subroutine 440 finds the R wavepeak times T.sub.R(j) for the 1.sup.st through (N+2).sup.th beat (j=1through N+2) in the baseline electrogram segment saved in step 439. Thisis a total of N+2 beats. Each time T.sub.R(j) is typically counted fromthe beginning of the Y second long electrogram segment until the peak ofthe j′th R wave.

Next in step 442 the average R-R interval of the i′th baselineelectrogram segment RR(i) is calculated by averaging the R-R intervalsfor each of the N+1 beats (j=2 through N+2) where the R-R interval forbeat j is T.sub.R(j)-T.sub.R(j−1). For example, for beat 2, the R-Rinterval is the time interval from the R wave peak of beat 1 (the veryfirst R wave) to the R wave peak of beat 2. I.e. R-R intervals beforeand after each of the N beats j=2 through j=N+1 are calculated. Thisstep also identifies any R-R intervals that are out of the “normal”range as defined in the programming of the cardiosaver 5. In a preferredembodiment of the present invention, baseline data will only beextracted from “normal” beats. A normal beat is one in which the R-Rinterval both before and after the R wave is in the “normal range. Thisis a preferred technique to use as a too short R-R interval before the Rwave can affect the PQ segment amplitude and a too short R-R intervalafter the R wave can affect the ST segment amplitude, either of whichcould produce a false indication of excessive ST shift.

Next in step 443 the offsets T.sub.PQ, T.sub.ST, D.sub.PQ and D.sub.ST(see FIG. 6) are calculated. In one embodiment, T.sub.PQ and T.sub.STare the percentages phi.sub.PQ and phi.sub.ST multiplied by the averageR-R interval RR(i) respectively. This technique will adjust the locationof the start of the PQ and ST segments to account for changes in heartrate. The percentages phi.sub.PQ and .phi . . . sub.ST would be selectedby the patient's doctor based on “normal” electrogram segments analyzedby the programmer 68 of FIG. 1. Another embodiment of the presentinvention uses fixed time offsets T.sub.PQ and T.sub.ST that areprogrammed by the patient's doctor. Similarly the duration of the PQ andST segments D.sub.PQ and D.sub.ST (see FIG. 6) can be calculated bymultiplying the percentages .delta . . . sub.PQ and .delta.sub.ST timesthe average R-R interval RR(i) respectively. The percentages.delta.sub.PQ and .delta.sub.ST would also be selected by the patient'sdoctor using the programmer 68. The preferred embodiment of the presentinvention uses fixed segment durations D.sub.PQ and D.sub.ST that areprogrammed by the patient's doctor. Using fixed durations D.sub.PQ andD.sub.ST has the advantage of keeping the same number of samplesaveraged in each calculation of the average PQ and ST segment amplitudesV.sub.PQ and V.sub.ST respectively.

Next in step 444 for each of the N beats (j=2 through N+1) identified bystep 422 as a normal beat, V.sub.PQ(j) the average of the PQ segmentamplitude of the j′th beat over the duration D.sub.PQ beginning T.sub.PQbefore the peak T.sub.R(j) and V.sub.ST(j) the average ST segmentamplitude of the j′th beat over the duration D.sub.ST beginning T.sub.STafter the time T.sub.R(j) are calculated. Similarly, step 444 calculatesthe peak T wave heights V.sub.T(j).

For each beat the ST deviation .DELTA.V.sub.ST(j) that is the differencebetween V.sub.ST(j) and V.sub.PQ(j) is then calculated in step 445.Similarly, step 445 calculates the T wave deviation .DELTA.V.sub.T(j)that is the difference between V.sub.T(j) and V.sub.PQ(j). It should benoted that step 455 of FIG. 5 will only allow the baseline extractionsubroutine to be run if less than 2 too short beats are present, thus atleast N−2 of the N beats used for baseline data extraction will benormal beats. Although there is a limit here of less than 2 short beats,it is envisioned that other minimum numbers of short beats than 2 mightalso be used.

Next in step 446 the ST deviation .DELTA.V.sub.ST(j) for all normalbeats within the N beats is averaged to produce the i′th averagebaseline ST deviation .DELTA.V.sub.BASE(i). Similarly, in step 446 the Twave deviation .DELTA.V.sub.T(j) for all normal beats within the N beatsis averaged to produce the i′th average baseline T wave deviation.DELTA.T.sub.BASE(i).

An alternate embodiment of the present invention would also check forexcessive ST shift on each normal beat and exclude any such beats fromthe average baseline ST deviation and T wave deviation calculations.

Next in step 447, .DELTA.R(i) the average of the height of the peak ofthe j′th R wave above the average PQ segment V.sub.PQ(j) is calculatedfor the normal beats. .DELTA.R(i) acts as an indication of the averagesignal strength of the i′th baseline electrogram segment. .DELTA.R(i) isused to provide a detection threshold for excessive ST shift that willadapt to slow changes in electrogram signal strength over time. This isof most value following implant as the sensitivity of the electrodes 14and 17 may change as the implant site heals.

.DELTA.T.sub.BASE (i) can either be the average of the signal samples ofthe entire T waves or it can be the average of the peak amplitude of theT waves in the normal beats. It is also envisioned, that if both ST andT wave shift detection are used, a cardiac event could be declared ifeither excessive ST shift or T wave shift detects a change (this ispreferred) or the program could require that both excessive ST shift andT wave shift be present.

Next in step 448, the threshold for ST shift detection for normal heartrates H.sub.ST(i) is calculated by multiplying the programmed thresholdpercentage P.sub.ST of .DELTA.R(i). Also in step 448, if the T waveshift detector is being used, the threshold for T wave shift detectionfor normal heart rates H.sub.T(i) is calculated by multiplying theprogrammed threshold percentage P.sub.T of .DELTA.R(i).

Finally in step 449, the extracted baseline parameters.DELTA.V.sub.BASE(i), .DELTA.T.sub.BASE(i), .DELTA.R(i), H.sub.ST(i) andH.sub.T(i) are saved to the calculated baseline data memory 475. Thebaseline extraction subroutine 440 has ended and the program returns tothe main heart signal processing program 450 step 451 of FIG. 5.

One embodiment of ST shift and T wave shift detection might use abaseline for ST shift detection that is 24.+−.½ hour before and abaseline for T wave shift that is 1 to 4 minutes in the past. This wouldrequire that the baseline extraction subroutine 440 be run for T waveshift parameters approximately every 60 seconds and for ST segmentparameters every hour.

Although the baseline extraction subroutine 440 is described here asusing the same “N” as the number of beats processed as the ST shiftdetection steps 457 and 469 of FIG. 5, it is envisioned that either agreater or lesser number of beats could be used for baseline extractionas compared with the number of beats “N” checked for excessive ST shiftsin FIG. 5.

Typical values used for the baseline extraction subroutine 440 as shownin FIG. 7 would be N=8 to average the data over 8 beats using beats 2through 9 of the Y second long electrogram segment. However, it isenvisioned that as few as 1 beat or as many as 100 beats or higher couldbe used to calculate the parameters extracted by subroutine 440. Alsoeven though the preferred embodiment of the present invention extractsbaseline data only from “normal” beats, it is envisioned that using all8 beats would usually yield an acceptable result.

Although the baseline extraction subroutine 440 shows the extraction ofparameters for identifying excessive ST shifts and T wave shifts, thecardiosaver 5 would function with either of these detection methods orcould use other techniques to measure the changes in electrogram signalsindicating one or more coronary event.

FIG. 8 illustrates a preferred embodiment of the alarm subroutine 490.The alarm subroutine 490 is run when there have been a sufficient numberof events detected to warrant a major event cardiac alarm to thepatient. The alarm subroutine 490 begins with step 491 where the entirecontents of both baseline electrogram memory 474 and recent electrogrammemory 472 of FIG. 4 are saved into the event memory 476. This saves theabove mentioned electrogram data in a place where it is not overwrittenby new baseline or recent electrogram data to allow the patient'sphysician to review the electrogram segments collected during a periodof time that occurred before the alarm. In a preferred embodiment with24 baseline electrogram segments collected once per hour, and 8 recentelectrogram segments collected every 30 seconds, the physician will beable to review a significant amount of electrogram data from the 4minutes just before the cardiac event as well as being able to see anychanges in the 24 hours before the event.

Next; in step 492 the internal alarm signal is turned on by having theCPU 44 of FIG. 4 cause the alarm sub-system 48 to activate a major eventalarm signal.

Next in step 493 the alarm subroutine instructs the CPU 44 to send amajor event alarm message to the external alarm system 60 of FIG. 1through the telemetry sub-system 46 and antenna 35 of the cardiosaver 5of FIG. 4. The alarm message is sent once every L1 seconds for L2minutes. During this time step 494 waits for an acknowledgement that theexternal alarm has received the alarm message. After L2 minutes, if noacknowledgement is received, the cardiosaver 5 of FIG. 1 gives up tryingto contact the external alarm system 60. If an acknowledgement isreceived before L2 minutes, step 495 transmits alarm related data to theexternal alarm system. This alarm related data would typically includethe cause of the. alarm, baseline and last event electrogram segmentsand the time at which the cardiac event was detected.

Next in step 496, the cardiosaver 5 transmits to the external alarmsystem 60 of FIG. 1 other data selected by the patient's physician usingthe programmer 69 during programming of the cardiosaver. These data mayinclude the detection thresholds H.sub.ST(i), H.sub.T(i) and otherparameters and electrogram segments stored in the cardiosaver memory 47.

Once the internal alarm signal has been activated by step 492, it willstay on until the clock/timing sub-system 49 of FIG. 4 indicates that apreset time interval of L3 minutes has elapsed or the cardiosaver 5receives a signal from the external alarm system 60 of FIG. 1 requestingthe alarm be turned off.

To save power in the implantable cardiosaver 5, step 496 might checkonce every minute for the turn off signal from the external alarm system60 while the external alarm system 60 would transmit the signalcontinuously for slightly more than a minute so that it will not bemissed. It is also envisioned that when the alarm is sent to theexternal alarm system 60, the internal clock 49 of the cardiosaver 5 andthe external alarm system 60 can be synchronized so that the programmingin the external alarm system 60 will know when to the second, that thecardiosaver will be looking for the turn off signal.

At this point in the alarm subroutine 490 step 497 begins to record andsave to event memory 476 of FIG. 4, an E second long electrogram segmentevery F seconds for G hours, to allow the patient's physician and/oremergency room medical professional to read out the patient'selectrogram over time following the events that triggered the alarm.This is of particular significance if the patient, his caregiver orparamedic injects a thrombolytic or anti-platelet drug to attempt torelieve the blood clot causing the acute myocardial infarction. Byexamining the data following the injection, the effect on the patientcan be noted and appropriate further treatment prescribed.

In step 498 the alarm subroutine will then wait until a reset signal isreceived from the physician's programmer 68 or the patient operatedinitiator 55 of the external alarm system 60 of FIG. 1. The reset signalwould typically be given after the event memory 476 of FIG. 4 has beentransferred to a component of the external equipment 7 of FIG. 1. Thereset signal will clear the event memory 476 (step 499) and restart themain program 450 at step 451.

If no reset signal is received in L6 hours, then the alarm subroutine490 returns to step 451 of FIG. 5 and the cardiosaver 5 will once againbegin processing electrogram segments to detect a cardiac event. Ifanother event is then detected, the section of event memory 476 used forsaving post-event electrogram data would be overwritten with thepre-event electrogram data from the new event. This process willcontinue until all event memory is used. I.e. it is more important tosee the electrogram data leading up to an event than the data followingdetection.

FIG. 9 illustrates the function of the hi/low heart rate subroutine 420.The hi/low heart rate subroutine is meant to run when the patient'sheart rate is below the normal range (e.g. 50 to 80 beats per minute) orabove the elevated range that can occur during exercise (e.g. 80 to 140beats per minute). A low heart rate (bradycardia) may indicate the needfor a pacemaker and should prompt a SEE DOCTOR alert to the patient ifit does not go away after a programmed period of time. Very high heartrate can be indicative of tachycardia or ventricular fibrillation and isserious if it does not quickly go away and should warrant a major eventalarm like a detected AMI.

The hi/low heart rate subroutine 420 begins with step 421 where theelectrogram segment of Y seconds collected in steps 453 and 454 of FIG.5 is saved to the event memory 476 (step 421) because the patient'sdoctor may wish to know that the high or low heart rate occurred. Oncethe Y second long electrogram segment is saved, step 422 of the hi/lowheart rate subroutine 420 directs the processing in different directionsdepending on if the heart rate is too high, too low or unsteady. Ifunsteady, the unsteady heart rate subroutine 410 illustrated in FIG. 12is run. If it is too high, step 423 increments the event counter k by 1,then step 424 checks whether the event counter k is equal to 3. Althoughthis embodiment uses k=3 events as the trigger to run the alarmsubroutine 490 it is envisioned that k=1 or 2 or k values higher than 3can also be used.

In step 424, If k=3 then the alarm subroutine 490 illustrated in FIG. 8is run. If k less than 3 then in step 425 the hi/low heart ratesubroutine 420 waits for “B” seconds and checks again in step 426 if theheart rate is still too high. If the heart rate is still too high, thehi/low heart rate subroutine 420 returns to step 423 where the eventcounter is incremented by 1. If the heart rate remains high, the hi/lowheart rate subroutine 420 will loop until k is equal to 3 and the alarmsubroutine 490 is run. If the heart rate does not remain high in step426, the hi/low heart rate subroutine 420 will return to step 453 of themain heart signal processing program 450 illustrated in FIG. 5. ST shiftamplitude (and/or T wave shift) is not checked during the high heartrate section of the hi/low heart rate subroutine 420 as the presence ofa very high heart rate could alter the detection of changes in ST and PQsegments of the electrogram giving false indications. Very high heartrate is, by itself, extremely dangerous to the patient and is thereforea major cardiac event.

If in step 422, the heart rate is too low rather than too high, thehi/low heart rate subroutine 420 will proceed to step 431 where the Ysecond long electrogram segment is checked for an excessive ST shift inthe same way as step 457 of the main heart signal processing program 450illustrated in FIG. 5. In other words, the ST deviation on M out of Nbeats must be shifted at least H.sub.ST(i) from the baseline average STdeviation .DELTA.V.sub.BASE(i) of the i′th baseline electrogram segment.If there is a detected excessive ST shift in step 431, the hi/low heartrate subroutine 420 returns to run the ST shift verification subroutine460 illustrated in FIG. 5. As with step 457 of the main heart signalprocessing program 450, the detection of M−N+1 OK beats withoutexcessive ST shift is sufficient for a negative detection and theprogram can then proceed on to step 432.

If there is not an excessive ST shift detected in step 431, step 432causes the hi/low heart rate subroutine 420 in step 432 to wait for “C”seconds then buffer and save a new Y second long electrogram segment asin steps 453 and 454 of the main heart signal processing program 450 ofFIG. 5. Once the new Y second long electrogram segment is collected, thehi/low heart rate subroutine 420 checks in step 433 if the heart rate isstill too low. If it is no longer too low, the system returns to step455 of the main heart signal processing program 450 illustrated in FIG.5.

If the heart rate remains too low, then step 434 checks for an excessiveST shift as in step 431. If there is an excessive ST shift in step 434,the hi/low heart rate subroutine 420 returns to run the ST shiftverification subroutine 460 of FIG. 5. If there is not an excessive STshift detected in step 434, step 435 causes the hi/low heart ratesubroutine 420 in step 435 to wait for another “C” seconds then bufferand save another Y second long electrogram segment as in steps 453 and454 of the main heart signal processing program 450 of FIG. 5. Once thisY second long electrogram segment is collected, the hi/low heart ratesubroutine 420 checks in step 436 if the heart rate is still too low(for the 3.sup.rd time). If it is no longer too low, the system returnsto step 455 of the main heart signal processing program 450 of FIG. 5.If the heart rate remains too low, then step 437 checks for an excessiveST shift as in steps 431 and 434. If there is an excessive ST shift instep 437, the hi/low heart rate subroutine 420 returns to run the STshift verification subroutine 460 of FIG. 5. If there is not anexcessive ST shift detected in step 437, the step 438 saves the contentsof the most recently collected Y second long electrogram segment and theto the event memory 476 for later review by the patient's doctor.

If the hi/low heart rate subroutine 420 reaches step 438 then thepatient's heart rate has been too low even after two waits of “C”seconds. Now the hi/low heart rate subroutine 420 proceeds to step 427to turn on the internal “SEE DOCTOR” alarm signal. Step 427 also sendsout to the external alarm system 60 of FIG. 1, a signal to activate the“SEE DOCTOR” alarm signal of the external alarm system 60 that mayinclude a text or played speech message indicating the cause of thealarm. E.G. the external alarm system speaker 57 of FIG. 1 could emitwarning tones and a text message could be displayed or the speaker 57might emit a spoken warning message to the patient.

Note that during the checking for continued low heart rate, ST shiftamplitudes are still checked after each wait because it is well knownthat low heart rate can be a byproduct of an acute myocardialinfarction.

Finally in step 428, the hi/low heart rate subroutine 420 will keep the“SEE DOCTOR” alarm signal turned on for L4 minutes or until receipt of asignal from the external alarm system 60 to turn off the alarm signal.After the “SEE DOCTOR ALERT signal is enabled, the low heart rate limit,below which the hi/low heart rate subroutine 420 is run, is changed bystep 429 to be just below the average heart rate measured in step 436.Once the patient is warned to go see the doctor, additional warningswill be annoying and therefore the low rate limit is best changed. Thisallows the hi/low heart rate subroutine 420 to then return to step 452of the main program where it will continue to monitor ST shiftamplitudes to provide early detection of acute myocardial infarction.Actual programming of the cardiosaver 5 may use R-R interval instead ofheart rate and it is understood that either is sufficient and one can beeasily computed from the other.

Although steps 431, 434 and 437 indicate the subroutine 420 is to lookfor an ST shift, other ischemia indications such as T wave spiking,either alone or in combination with ST shift detection may be used. Alsoin steps 431, 434 and 437 if no shift is detected, the event counter kis reset to 0 if it is not already 0.

FIG. 10 illustrates the ischemia subroutine 480 that provides decisionmaking for the cardiosaver 5 in the event of an elevated heart rate suchas that would occur during exercise by the patient. The ischemiasubroutine 480 uses a beat counter j to indicate the beat within a Ysecond long electrogram segment. A beat is defined as a sub-segmentcontaining exactly one R wave of the Y second long electrogram segment.The ischemia subroutine 480 begins in step 481 by initializing the beatcounter j to a value of 2. Then in step 482, the R-R interval range Afor the beat j is determined. For example that there could be between 4R-R interval ranges A=1 to 4 of 750 to 670, 670 to 600, 600 to 500 and500 to 430 milliseconds respectively. These would correspond to heartrate intervals of 80 to 90, 90 to 100, 100 to 120 and 120 to 140 beatsper minute. The number of ranges A and the upper and lower limit of eachrange would be programmable by the patient's physician from theprogrammer 68 of FIG. 1.

Next in step 483 the programmed ischemia multiplier .mu.(A) is retrievedfrom the programmable parameters 471 of FIG. 4. .mu.(A) is the allowablefactor increase or decrease in ST shift detection threshold for the R-Rinterval range A. In other words, because the patient may have someischemia during elevated heart rates from exercise, the patient'sphysician can program .mu.(A)s that are greater than 1 and mightincrease with each successive heart rate range. For example, if the R-Rinterval ranges are 750 to 670, 670 to 600, 600 to 500 and 500 to 430milliseconds the corresponding .mu.(A)s might be 1.1, 1.2, 1.3 and 1.5.This would require that the ST shift in the R-R interval range of A=4(500 to 430 milliseconds) be one and a half times as large as duringnormal heart rates in order to qualify as a cardiac event. It isenvisioned that the patient could undergo an exercise stress test at atime after implant when the implanted leads have healed into the wall ofthe heart and electrogram segments captured by the cardiosaver 5 duringthat stress test would be reviewed by the patient's physician todetermine the appropriate range intervals and ischemia multipliers tohelp identify a worsening of the patient's exercise induced ischemiafrom the time when the stress test is conducted.

It is also envisioned that in order to detect smaller changes in vesselnarrowing than a full acute myocardial infarction, the cardiosaver S ofFIGS. 1-4 might use .mu.(A)s that are less than one. For example, if theR-R interval ranges are 750 to 670, 670 to 600, 600 to 500 and 500 to430 milliseconds the corresponding .mu.(A)s might be 0.5, 0.6, 0.7 and0.8. Thus in this example, in the R-R interval range of 750 to 670milliseconds, the threshold for ischemia detection would be half of whatit is for the normal heart rate range.

Once the ischemia multiplier has been retrieved, step 484 calculates theischemia ST shift threshold .theta.(A) for the R-R interval range Awhere .theta.(A)=H.sub.ST(i).times.mu.(A) where H.sub.ST(i) is thecurrent ST shift threshold for normal heart rates. Next in step 485, theischemia subroutine 480 checks if for the beat j the ST shift is greaterthan the ischemia threshold .theta.(A). If it is not greater, step 487then checks if the N′th beat has been examined. If the ST shift of thej′th beat exceeds the ischemia threshold .theta.(A) then step 486 checksif M beats with ST shifts greater than theta.(A) have been seen. If theyhave not been seen proceed to step 487. If in step 487, the Nth beat hasbeen examined, return to step 451 of the main heart signal processingprogram 450 of FIG. 5. If N beats have not yet been examined, incrementj by 1 in step 489 and loop back to step 482.

If M beats with excessive ST shift are found by step 486, step 581 savesthe current Y second long electrogram segment to the Event Memory 476,then in step 582 the event counter k is incremented by I followed bystep 583 checking if k is equal to 3. If k is less than 3 then theischemia subroutine 480 continues by sleeping for Z seconds in step 584,then buffering a new Y second long electrogram segment in step 585,saving in step 586 the new Y second long electrogram segment to the nextlocation in recent electrogram memory 472 of FIG. 4. and then checkingif the heart rate is still elevated in step 587. If the heart rate isstill elevated in step 587, the loop checking for ischemia is run againstarting with step 481. If the heart rate is no longer elevated thenstep 588 checks if the heart rate is too high, too low or unsteady. Ifsuch is the case, the hi/low heart rate subroutine 420 is run. If theheart rate is not high, low or unsteady, the ischemia subroutine 480ends and the program returns to step 469 of the ST shift verificationsubroutine 460 of FIG. 5. This will allow an excessive ST shift detectedat elevated heart rate that stays shifted when the heart rate returns tonormal to quickly trigger the AMI alarm. This works because k is either1 or 2 at this point so either 2 or I more detection of excessive STshift with normal heart rate will cause a major event AMI alarm. Ifhowever k=3 in step 582, then the last detection of excessive ST shiftoccurred during an elevated heart rate and will be treated as exerciseinduced ischemia rather than an acute myocardial infarction.

So if k=3 (i.e. exercise induced ischemia has been detected) in step 582the ischemia subroutine 480 moves on to step 681 where it checks if ithas been more than L5 minutes since the first time that exercise inducedischemia was detected where k=3 in step 583.

If it has been less than L5 minutes since the first detection ofexercise induced ischemia then the internal SEE DOCTOR ALERT signal isturned on by step 682 if it has not already been activated.

If it has been more than L5 minutes, then the alarm subroutine 490 isrun. This will change the SEE DOCTOR ALERT signal previously started instep 682 to a major event AMI alarm if the excessive ST shift at anelevated heart rate does not go away within L5 minutes. Similarly, ifthe patient stops exercising and his heart rate returns to normal butthe excessive ST shift remains, then the alarm subroutine 490 will alsobe run.

If it has been less than L5 minutes and the SEE DOCTOR alert signal hasnot been already been activated, step 683 next sends a message to theexternal alarm system 60 of FIG. 1 to activate the SEE DOCTOR externalalarm signal and indicate to the patient by a text of spoken messagethat he should stop whatever he is doing, and sit or lie down to get hisheart rate to return to normal. Following this, in step 684 the ischemiasubroutine 480 will keep the SEE DOCTOR ALERT signal on for L4 minutesfrom the first time it is turned on or until the receipt of an offsignal from the alarm disable button 59 of the external alarm system 60of FIG. 1. The program then returns to step 451 of the main program 451of FIG. 5 to continue to examine the patient's heart signals.

FIG. 11 diagrams the alarm conditions 600 that are examples of thecombinations of major and minor events that can trigger an internalalarm signal (and/or external alarm signal for the guardian system ofFIG. 1. Box 610 shows the combinations 611 through 617 of major cardiacevents that can cause the alarm subroutine 490 to be run. These includethe following:

611. 3 ST shift events (detections of excessive ST shift) with either anormal heart rate or a low heart rate.

612. 2 ST shift events with a normal or low heart rate and 1 event fromheart rate too high.

613. 1 ST shift event with a normal or low heart rate and 2 events fromheart rate too high.

614. 3 events from heart rate too high.

615. 3 ST shift events with either a normal, low or elevated heart rate(ischemia) where the last detection is at a normal or low heart rate.

616. 3 events (excessive ST shift or high heart rate) where the lastevent is high heart rate.

617. An ischemia alarm indication from conditions in box 620 thatremains for more than L5 minutes after the first detection of ischemia.

The ischemia alarm conditions 620 include:

621. 3 ST shift events with either a normal, low or elevated heart rate(ischemia) where the last detection is at an elevated heart rate.

622. Any 3 events including a too high heart rate event where the lastdetection is an excessive ST shift at an elevated heart rate.

If either of the ischemia alarm conditions 620 is met and it is lessthan L5 minutes since the exercise induced ischemia was first detected,then the SEE DOCTOR ALERT signal will be turned on by step 682 of theischemia subroutine 480 if it has not already been activated.

Box 630 shows the other minor event alarm conditions including thebradycardia alarm condition 632 that is three successive electrogramsegments collected with heart rate too low and the unsteady heart ratealarm condition 635 that is caused by more than P.sub.unsteady % ofbeats having a too short R-R interval. If here are too many (asprogrammed by the doctor) consecutive electrogram segments withinsufficient normal beats 637 to be able to process for cardiac eventdetection, the programming may need modification or there is somethingelse wrong. These will trigger the SEE DOCTOR alert signal initiated bystep 427 of the hi/low heart rate subroutine 420 for the bradycardiaalarm condition 632 and step 416 of the unsteady hart rate subroutine410 for the unsteady heart rate alarm condition 635. Also triggering theSEE DOCTOR alert signal is a low battery condition 636.

FIG. 12 is a block diagram illustrating the unsteady heart ratesubroutine 410. The subroutine 410 is run if the R-R interval variesgreatly over many of the beats in the Y second long electrogram segmentcollected by steps 453 and 454 of the main heart signal processingprogram 450. As previously described, one technique for identifying suchan unsteady heart rate is to compare the two shortest R-R intervals andthe 2 longest intervals. If the difference between the both of the twoshortest and the average of the two longest R-R intervals are more thana programmed percentage a, an unsteady heart rate is identified. Forexample the programmed percentage .alpha. might be 25% so that if thetwo shortest R-R intervals are each more than 25% less than the averageof the two longest R-R intervals, then the heart rate is unsteady. It isenvisioned that if a longer time Y is used for electrogram segmentcollection then it might require 3 or more “short” beats to indicated anunsteady heart rate. If there is zero or one short beat, the main heartsignal processing program 450 will move on to step 456 having marked allof the “normal” beats in the Y second long electrogram segment. A normalbeat is defined as a beat including where the R-R intervals before andafter the R wave are both in the normal range (i.e. not too short).

The unsteady heart rate subroutine 410 begins in step 411 by checkingfor at least N normal beats in the most recently collected electrogramdata. When the subroutine begins there is only one Y second longelectrogram segment being examined. If there are not N normal beats,then the subroutine 410 will wait X seconds in step 419 before anadditional Y second long electrogram segment is collected in step 412after the . Step 411 then will check for N normal beats in the two Ysecond long electrogram segments (i.e. 2Y seconds of electrogram data).This loop of steps 411 and 412, where each time Y additional seconds ofelectrogram is collected, will continue until N normal beats are found.

It is envisioned that step 411 could also check for beats with elevatedheart rate R-R intervals or might include elevated heart rate beats as“normal” beats by expanding the allowed range of the R-R interval for anormal beat. Once N “normal” beats are found by step 411, then step 413checks for an excessive ST shift in M out of the N normal beats similarto step 457 of FIG. 5. Step 413 could also (as in step 457 of FIG. 5)look for an excessive T wave shift. If an excessive ST shift (and/or Twave shift) is detected by step 413, the program returns to the ST shiftverification subroutine 460 of FIG. 5.

If excessive ST shift (and/or T wave shift) are not detected by step413, then step 414A checks if more than P.sub.unsteady % of all thebeats (not just the normal beats) in the electrogram data collected havea too short R-R interval as defined above by the programmed parameter a.If not the program returns to step 451 of the main heart signalprocessing program 450 of FIG. 5. If, however, more than P.sub.unsteady% of the beats have a short R-R interval, then step 414B ascertains ifthere have been N.sub.u sequential electrogram segments having more thanP.sub.unsteady % of the beats with short R-R intervals. If the number isless than Nu then this then the program returns to step 451 of the mainheart signal processing program 450 of FIG. 5. If the number is Nu thenstep 415 saves all the current electrogram data to event memory 476 ofFIG. 4 and step 416 turns on the SEE DOCTOR alert signal with theinternal alarm sub-system 48 of FIG. 4 and also initiates an externalalarm signal by the external alarm system 60 of FIG. 1 with a text orspoken message to the patient indicating that the SEE DOCTOR alertsignal is the result of detection of unsteady heart rate. As in the caseof other SEE DOCTOR alert signals, step 417 will keep the “See Doctor”alarm mechanism turned on for L4 minutes from the first detection ofunsteady heart rate or until receipt of a signal from the external alarmsystem 60 to turn off the alarm. To avoid continuously alarming thepatient, once the SEE DOCTOR alert has sounded, the system will wait fora preset time programmed by the patient's physician before allowingreactivation of the SEE DOCTOR ALERT. Alternately, there may be adefault wait period such as 12 hours or 1 day or the system may beprogrammed to only sound the SEE DOCTOR alert once for each indicationuntil reset by the physician's programmer.

FIG. 13 shows a modified embodiment of the guardian system 510. Thecardiosaver implant 505 with lead 512, electrode 514, antenna 516,header 520 and metal case 511 would be implanted subcutaneously in apatient at risk of having a serious cardiac event such as an acutemyocardial infarction. The lead 512 could be placed eithersubcutaneously or into the patient's heart. The case 511 would act asthe indifferent electrode. The system 510 also included externalequipment that includes a physician's programmer 510 an external alarmtransceiver 560 and a pocket PC 540 with charger 566. The external alarmtransceiver 560 has its own battery 561 and includes an alarm disablebutton 562 radiofrequency transceiver 563, speaker 564, antenna 565 andstandard interface card 552. The cardiosaver 505 has the samecapabilities as the cardiosaver 5 of FIGS. 1 through 4.

The standardized interface card 552 of the external alarm transceiver510 can be inserted into a standardized interface card slot in ahandheld or laptop computer. The pocket PC 540 is such a handheldcomputer. The physician's programmer 510 is typically a laptop computer.Such standardized card slots include compact flash card slots, PCMCIAadapter (PC adapter) card slots, memory stick card slots, Secure Digital(SD) card slots and Multi-Media card slots. The external alarmtransceiver 510 is designed to operate by itself as a self-containedexternal alarm system, however when inserted into the standardized cardslot in the pocket PC 540, the combination forms an external alarmsystem with enhanced functionality. For example, in stand alone modewithout the pocket PC 540, the external alarm transceiver 560 canreceive alarm notifications from the cardiosaver implant 505 and canproduce an external alarm signal by generating one or more soundsthrough the speaker 564. These sounds can wake the patient up or provideadditional alerting to that provided by the internal alarm signalgenerated by the cardiosaver 505. The alarm disable button 562 canacknowledge and turn off both external and internal alarm signals. Thestandalone external alarm transceiver 560 therefore provides keyfunctionality could be small enough to wear on a chain around the neckor on a belt.

When plugged into the pocket PC 540, the external alarm transceiver 560can facilitate the display of text messages to the patient andelectrogram data that is transmitted from the cardiosaver 505. Thepocket PC 540 also enables the patient operated initiator 55 and panicbutton 52 capabilities of the external alarm system 60 of FIG. 1. Beinga pocket PC also readily allows connection to wireless communicationcapabilities such as wireless internet access that will facilitateretransmission of data to a medical practitioner at a geographicallyremote location. It is also envisioned that the charger 566 couldrecharge the batter 551 when the external alarm adaptor 560 is pluggedinto the pocket PC 540.

The external alarm transceiver 560 can also serve as the wirelesstwo-way communications interface between the cardiosaver 505 and theprogrammer 510. The physician's programmer 510 is typically a laptopcomputer running some version of the Microsoft Windows operating system.As such, any or the above standardized slot interfaces can be eitherdirectly interfaced to such a laptop computer or interfaced using areadily available conversion adaptor. For example, almost all laptopcomputers have a PCMCIA slot and PCMCIA card adaptors are available forcompact flash cards, Secure Digital cards etc. Thus the external alarmadaptor 560 could provide the interface to the physician's programmer510. This provides additional security as each cardiosaver implant 505and external alarm adaptor 560 could be uniquely paired with built insecurity codes so that to program the implant 505, the physician wouldneed the patient's external alarm adaptor 560 that would act both as awireless transceiver and as a security key.

Although the guardian system 10 as described herein could clearlyoperate as a stand-alone system, it is clearly conceivable to utilizethe guardian system 10 with additional pacemaker or implanteddefibrillator circuitry. As shown in FIG. 4, pacemaker circuitry 170and/or defibrillator circuitry 180 could be made part of any cardiosaver5 or 505. Furthermore, two separate devices (one pacemaker or onedefibrillator plus one cardiosaver 5) could be implanted within the samepatient.

FIG. 14 illustrates a preferred physical embodiment of the externalalarm transceiver 560 having standardized interface card 552, alarmdisable button 562 labeled “ALARM OFF” and speaker 564. It is alsoenvisioned that by depressing and holding the alarm disable button 562for a minimum length of time, when there is not an alarm, the externalalarm transceiver could verify the operational status of the cardiosaver505 and emit a confirming sound from the speaker 564.

FIG. 15 illustrates the physical embodiment of the combined externalalarm transceiver 560 and pocket PC 540 where the standardized interfacecard 552 has been inserted into a matching standardized interface cardslot the pocket PC 540. The screen 542 of the pocket PC 540 shows anexample of the display produced by an external alarm system followingthe detection of an acute myocardial infarction by the cardiosaver 505.The screen 542 of FIG. 15 displays the time of the alarm, the recentelectrogram segment from which the cardiac event was detected and thebaseline electrogram segment used for comparison in the cardiac eventdetection. Such a display would greatly facilitate diagnosis of thepatient's condition upon arrival at an emergency room and couldeliminate the need for additional electrocardiogram measurements beforethe patient is treated.

FIG. 16 shows and advanced embodiment of the external alarm transceiver720 having a battery 721, an alarm disable button 722, a RF transceiverfor data communication to and from the implanted device, a loudspeaker724, a microphone 727, a local area wireless interface 723, a standardinterface 728 and a long distance (LD) voice/data communicationinterface 729. The function of the alarm disable button 722 and theradiofrequency transceiver 723 are as described for the similar devicesshown in FIG. 13.

The local area wireless interface 723 provides wireless communicationwithin a building (e.g. home, doctor's office or hospital) to and fromthe implant 505 with lead 512 and antenna 516 through the external alarmtransceiver 720 from and to assorted external equipment such as

Pocket PCs 702, Palm OS PDAs, Notebook PCs, physician's programmers 704and tablet diagnostic systems 706. The means for transmission from thelocal area wireless interface 723 may be by radiofrequency or infra-redtransmission. A preferred embodiment of the local area wirelessinterface 723 would use a standardized protocol such as IRDA withinfra-red transmission and Bluetooth or WiFi (802.11.a, b, or g) withradiofrequency transmission. The local area wireless interface 723 wouldallow display of implant data and the sending of commands to the implant505.

The standard interface 728 provides a physical (wired) connection fordata communication with devices nearby to the patient for the purposesof displaying data captured by the implant 505 and for sending commandsand programs to the implant 505. The standard interface 728 could be anystandard computer interface; for example: USB, RS-232 or parallel datainterfaces. The pocket PC 702 and physician's programmer 704 would havefunctionality similar to the pocket PC 540 and physician's programmer510 of FIG. 13.

The tablet diagnostic system 706 would provide a level of functionalitybetween that of the pocket PC 702 and physician's programmer 706. Forexample, the tablet diagnostic system would have the programmer'sability to download complete data sets from the implant 505 while thepocket PC is limited to alarm and baseline electrogram segments or themost recent electrogram segment. The tablet diagnostic system 706 wouldbe ideal for an emergency room to allow emergency room medicalprofessionals to quickly view the electrogram data stored within theimplant 505 to assess the patient's condition. The recently introducedTablet PCs such as the Toshiba Portege 3500 or the Compaq TC1000 haveIRDA, WiFi and USB interfaces built into them and so would make an idealplatform for the tablet diagnostic system 706. It is envisioned thatsuch a tablet diagnostic system in an emergency room or medical clinicwould preferably be connected to its own external alarm transceiver. Thetablet diagnostic system 706 could be hand held or mounted on a wall orpatient bed. A unit located near the bed of an incoming patient having aguardian implant 505 would enable display of patient diagnostic datawithout requiring any attachments to the patient. Such wirelessdiagnosis is similar to that envisioned for the tricorder and diagnosticbeds of the Star Trek science fiction series created by GeneRoddenberry.

The long distance voice/data communication interface 729 with microphone727 and also attached to the loudspeaker 724 will provide the patientwith emergency contact with a remote diagnostic center 708. Such asystem could work much like the ONSTAR emergency assistance system nowbuilt into many cars. For example, when a major or EMERGENCY alarm isidentified by the guardian implant 505, the following steps could befollowed:

1. The guardian will first ascertain if an external alarm transceiver iswithin range, if not the internal alarm will be initiated.

2. If the external alarm transceiver is within range the system willnext see if there is access to the remote diagnostic center 708 throughthe long distance voice/data communication interface 729. If not theexternal alarm transceiver 720 and implant 505 will initiate internaland/or external alarm notification of the patient.

3. If there is access to the remote diagnostic center 708 the longdistance voice/data communication interface 729, the patient alarminformation including alarm and baseline electrogram segments will betransmitted to the remote diagnostic center 708. A medical professionalat the remote diagnostic center 708 will view the data and immediatelyestablish voice communication to the external alarm transceiver 720through the long distance voice/data communication interface 729. Ifthis occurs, the first thing that the patient will hear is a ringingtone and/or a voice announcement followed by the contact with themedical professional who can address the patient by name and facilitateappropriate emergency care for the patient. In this case, the internaland external alarms will not be needed and to the patient it willresemble an incoming telephone call from the medical professional. It isalso envisioned that the voice of the medical professional could be thefirst thing that the patient hears although an initial alerting signalis preferred.

This method of establishing the highest level of communication availableto the guardian system with the fall back of just the internal alarmwill provide the best possible patient alerting based on what isavailable at the time of the alarm.

The data communications between the external alarm transceiver 720 andthe remote diagnostic center 708 would utilize a standardized (orcustom) data communications protocol. For example, the datacommunications might utilize any or all of the following either within aprivate network, a VPN, an intranet (e.g. a single provider network suchas the Sprint data network) or through the public internet:

1. Basic TCP/IP messaging within a single network or through theinternet.

2. Short Messaging Service (SMS)

3. Multimedia Message Service (MMS) used for cell phone transmission

4. Universal Datagram Protocol (UDP)

It is also envisioned that the present invention would take advantage ofexisting telephone network call center technology including use ofAutomatic Number Identification (ANI) to identify the incoming call, andDialed Number Identification Service (DNIS) where different numbersmight be dialed by the external alarm transceiver 720 depending on theseverity of the detected cardiac event. For example, in the case wherethe call is placed by the emergency alarm transceiver 720, an EMERGENCYalarm might dial a different number than a SEE DOCTOR alert which mightbe different from a patient-initiated “panic button” call. DNIS couldhelp get the appropriate help for the patient even if data connectivityis unavailable and might be used to prioritize which call is answeredfirst (e.g., an EMERGENCY alarm would have higher priority than a SEEDOCTOR alert).

It is also envisioned that the remote diagnostic center 708 couldfacilitate the scheduling of an appointment with the patient's doctorfollowing a SEE DOCTOR alert.

Although throughout this specification all patients have been referredto in the masculine gender, it is of course understood that patientscould be male or female. Furthermore, although the only electrogramindications for an acute myocardial infarction that are discussed hereinare shifts involving the ST segment and T wave height, it should beunderstood that other changes in the electrogram (depending on where inthe heart the occlusion has occurred and where the electrodes areplaced) could also be used to determine that an acute myocardialinfarction is occurring. Furthermore, sensors such as heart motionsensors, or devices to measure pressure, pO.sub.2 or any otherindication of an acute myocardial infarction or cardiac events could beused independently or in conjunction with a ST segment or T wave shiftdetectors to sense a cardiac event.

It is also envisioned that all of the processing techniques describedherein for an implantable cardiosaver are applicable to a guardiansystem configuration using skin surface electrodes and a non-implantedcardiosaver 5 the term electrogram would be replaced by the termelectrocardiogram. Thus the cardiosaver device described in FIGS. 5through 12 would also function as a monitoring device that is completelyexternal to the patient.

Various other modifications, adaptations, and alternative designs are ofcourse possible in light of the above teachings. Therefore, it should beunderstood at this time that, within the scope of the appended claims,the invention can be practiced otherwise than as specifically describedherein.

1. A system for identifying an ischemic event in a human patientincluding: at least two electrodes implanted in the patient forobtaining an electrical signal from the patient's heart; an implanteddevice including: (a) analog-to-digital converter circuitry fordigitizing the electrical signal to produce a multiplicity of segments,each having a duration of least 1 second, each of the periods betweensuccessive segments being at least as long as the segment duration; (b)memory means designed to store a first segment at a first predeterminedtime; (c) memory means designed to store a recently collected segment ata second time that is later than the first predetermined time; (d)processor means coupled to said memory means for comparing the STsegment voltage of at least one beat of the recently collected segmentat the second time with the average ST segment voltage of at least twobeats of the first segment stored in the memory means at the firstpredetermined time; and, (e) means for identifying that the ischemicevent has occurred by applying a test that is based on whether thedifference between the ST segment voltage of the at least one beat ofthe recently collected segment and the average ST segment voltage of theat least two beats of the first segment exceeds a detection threshold,wherein the detection threshold is a preset percentage of the averagesignal amplitude of the at least two beats of the first segment.
 2. Thesystem of claim 1 where the average signal amplitude of the at least twobeats of the first segment is the R wave peak voltage minus the averagePQ segment voltage averaged over the at least two beats of the firstsegment.
 3. The system of claim 1 where the average signal amplitude ofthe at least two beats of the first segment is the average peak-to-peak,R-to-S, signal amplitude of the at least two beats of the first segment.4. The system of claim 1 where the average signal amplitude of the atleast two beats of the first segment is the difference between theaverage PQ segment voltage and the peak S wave voltage averaged over theat least two beats of the first segment.
 5. The system of claim 1 wherethe average signal amplitude of the at least two beats of the firstsegment is the larger of the R wave to PQ segment voltage difference orthe PQ segment to S wave voltage difference.
 6. The system of claim 1wherein the first segment is a baseline segment.
 7. The system of claim1 wherein the test is based on whether the difference between the STsegment voltage of at least one beat of within a plurality of recentlycollected segments and the average ST segment voltage of the at leasttwo beats of the first segment exceeds the detection threshold.
 8. Thesystem of claim 1 wherein the processor is configured to compute thedifference between the ST segment voltage of a plurality of beats withinthe current electrogram segment and the average ST segment voltage ofthe at least two beats of the first segment, thereby generating aplurality of ST shifts, and wherein the test is based on the number ofthe plurality of ST shifts that exceed the detection threshold.
 9. Thesystem of claim 1 wherein the ST segment voltage is computed relative toPQ segment voltage.
 10. A system for identifying an ischemic event in ahuman patient including: at least two electrodes implanted in thepatient for obtaining an electrical signal from the patient's heart; animplanted device including: (a) analog-to-digital converter circuitryfor digitizing the electrical signal; (b) a memory; (c) a processorconfigured to: (i) compute ST segment deviation of a first plurality ofbeats within the electrical signal; (ii) compute a measure of QRSamplitude for the first plurality of beats; (iii) compute ST segmentdeviation of a second plurality of beats within the electrical signal,wherein the second plurality of beats occur after the first plurality ofbeats; (iv) compute a measure of the difference between the ST segmentdeviations of the first plurality of beats and the ST segment deviationsof the second plurality of beats; (v) apply an ischemia detection testthat is based on a comparison of the measure of the difference and athreshold, wherein the test is further based on the measure of QRSamplitude of the first plurality of beats.
 11. The system of claim 10wherein the threshold is a preset percentage effectively multiplied bythe measure of QRS amplitude.
 12. The system of claim 11 wherein theprocessor computes the threshold by multiplying the preset percentage bythe measure of QRS amplitude.
 13. The system of claim 10 wherein thedifference between the ST segment deviations of the first plurality ofbeats and the ST segment deviations of the second plurality of beats iscomputed separately for each of the second plurality of beats, therebygenerating a plurality of ST shifts, and wherein the ischemia test isbased on the number of said plurality of ST shifts that exceed thethreshold.
 14. The system of claim 10 wherein the processor is furtherconfigured to separate the electrical signal into a plurality ofsegments, each of the plurality of segments having a time duration ofleast 1 second, each of the periods between successive segments being atleast as long as the segment duration, and wherein the second pluralityof beats is distributed across a plurality of segments.
 15. The systemof claim 14 wherein each of the plurality of segments is categorized asischemic or non-ischemic, and wherein the test is positive for ischemiawhen a plurality of consecutive segments are categorized as ischemic.16. The system of claim 15 wherein the difference between the ST segmentdeviations of the first plurality of beats and the ST segment deviationsof the second plurality of beats is computed separately for each of thesecond plurality of beats, thereby generating a plurality of ST shifts,and wherein a segment is classified as ischemic or non-ischemicaccording to the number of ST shifts within the segment that exceed thethreshold.